**Describe the bug**
ConfigReloader tried to load users.xml and crashed
```
2021.01.26 19:34:29.532930 [ 50 ] {} <Debug> ConfigReloader: Loading config '/etc/clickhouse-server/users.xml'
```

``` xml
<?xml version="1.0"?>
<yandex>
    <!-- Profiles of settings. -->
    <profiles>
        <!-- Default settings. -->
        <default>
            <!-- Maximum memory usage for processing single query, in bytes. -->
            <max_memory_usage>10000000000</max_memory_usage>

            <!-- Use cache of uncompressed blocks of data. Meaningful only for processing many of very short queries. -->
            <use_uncompressed_cache>0</use_uncompressed_cache>

            <!-- How to choose between replicas during distributed query processing.
                 random - choose random replica from set of replicas with minimum number of errors
                 nearest_hostname - from set of replicas with minimum number of errors, choose replica
                  with minimum number of different symbols between replica's hostname and local hostname
                  (Hamming distance).
                 in_order - first live replica is chosen in specified order.
                 first_or_random - if first replica one has higher number of errors, pick a random one from replicas with minimum number of errors.
            -->
            <load_balancing>random</load_balancing>
        </default>

        <!-- Profile that allows only read queries. -->
        <readonly>
            <readonly>1</readonly>
        </readonly>
    </profiles>

    <!-- Users and ACL. -->
    <users>
        <!-- If user name was not specified, 'default' user is used. -->
        <default>
            <!-- Password could be specified in plaintext or in SHA256 (in hex format).

                 If you want to specify password in plaintext (not recommended), place it in 'password' element.
                 Example: <password>qwerty</password>.
                 Password could be empty.

                 If you want to specify SHA256, place it in 'password_sha256_hex' element.
                 Example: <password_sha256_hex>65e84be33532fb784c48129675f9eff3a682b27168c0ea744b2cf58ee02337c5</password_sha256_hex>
                 Restrictions of SHA256: impossibility to connect to ClickHouse using MySQL JS client (as of July 2019).

                 If you want to specify double SHA1, place it in 'password_double_sha1_hex' element.
                 Example: <password_double_sha1_hex>e395796d6546b1b65db9d665cd43f0e858dd4303</password_double_sha1_hex>

                 If you want to specify a previously defined LDAP server (see 'ldap_servers' in main config) for authentication, place its name in 'server' element inside 'ldap' element.
                 Example: <ldap><server>my_ldap_server</server></ldap>

                 How to generate decent password:
                 Execute: PASSWORD=$(base64 < /dev/urandom | head -c8); echo "$PASSWORD"; echo -n "$PASSWORD" | sha256sum | tr -d '-'
                 In first line will be password and in second - corresponding SHA256.

                 How to generate double SHA1:
                 Execute: PASSWORD=$(base64 < /dev/urandom | head -c8); echo "$PASSWORD"; echo -n "$PASSWORD" | sha1sum | tr -d '-' | xxd -r -p | sha1sum | tr -d '-'
                 In first line will be password and in second - corresponding double SHA1.
            -->
            <password></password>

            <!-- List of networks with open access.

                 To open access from everywhere, specify:
                    <ip>::/0</ip>

                 To open access only from localhost, specify:
                    <ip>::1</ip>
                    <ip>127.0.0.1</ip>

                 Each element of list has one of the following forms:
                 <ip> IP-address or network mask. Examples: 213.180.204.3 or 10.0.0.1/8 or 10.0.0.1/255.255.255.0
                     2a02:6b8::3 or 2a02:6b8::3/64 or 2a02:6b8::3/ffff:ffff:ffff:ffff::.
                 <host> Hostname. Example: server01.yandex.ru.
                     To check access, DNS query is performed, and all received addresses compared to peer address.
                 <host_regexp> Regular expression for host names. Example, ^server\d\d-\d\d-\d\.yandex\.ru$
                     To check access, DNS PTR query is performed for peer address and then regexp is applied.
                     Then, for result of PTR query, another DNS query is performed and all received addresses compared to peer address.
                     Strongly recommended that regexp is ends with $
                 All results of DNS requests are cached till server restart.
            -->
            <networks>
                <ip>::/0</ip>
            </networks>

            <!-- Settings profile for user. -->
            <profile>default</profile>

            <!-- Quota for user. -->
            <quota>default</quota>

            <!-- User can create other users and grant rights to them. -->
            <!-- <access_management>1</access_management> -->
        </default>
    </users>

    <!-- Quotas. -->
    <quotas>
        <!-- Name of quota. -->
        <default>
            <!-- Limits for time interval. You could specify many intervals with different limits. -->
            <interval>
                <!-- Length of interval. -->
                <duration>3600</duration>

                <!-- No limits. Just calculate resource usage for time interval. -->
                <queries>0</queries>
                <errors>0</errors>
                <result_rows>0</result_rows>
                <read_rows>0</read_rows>
                <execution_time>0</execution_time>
            </interval>
        </default>
    </quotas>
</yandex>
```
**Error message and/or stacktrace**
integration test_config_substitutions msan local run:
```
2021.01.26 19:34:29.538223 [ 54 ] {} <Fatal> BaseDaemon: ########################################
2021.01.26 19:34:29.538337 [ 54 ] {} <Fatal> BaseDaemon: (version 21.2.1.1, build id: 848D638C25A24123F0A66BCF1C9FC471B1BE106F) (from thread 50) (no query) Received signal Segmentation fault (11)
2021.01.26 19:34:29.538427 [ 54 ] {} <Fatal> BaseDaemon: Address: NULL pointer. Access: read. Address not mapped to object.
2021.01.26 19:34:29.538617 [ 54 ] {} <Fatal> BaseDaemon: Stack trace: 0x2c8ba60f 0x8f0c143 0x314ce5f4 0x314cd74a 0x31349875 0x3133218a 0x3132e08c 0x3132f179 0x3132f179 0x3132f179 0x3133c2ca 0x313423a2 0x31359253 0x3135d18d 0x31360594 0x31360379 0x9005977 0x9013ff8 0x7f4eecba1609 0x7f4eecac8293
2021.01.26 19:34:29.542451 [ 54 ] {} <Fatal> BaseDaemon: 4. ./obj-x86_64-linux-gnu/../src/Interpreters/Context.cpp:1560: DB::Context::getZooKeeper() const @ 0x2c8ba60f in /usr/bin/clickhouse
2021.01.26 19:34:29.545071 [ 54 ] {} <Fatal> BaseDaemon: 5. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/functional:2090: std::__1::shared_ptr<zkutil::ZooKeeper> std::__1::__function::__policy_invoker<std::__1::shared_ptr<zkutil::ZooKeeper> ()>::__call_impl<std::__1::__function::__default_alloc_func<DB::Server::main(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&)::$_2, std::__1::shared_ptr<zkutil::ZooKeeper> ()> >(std::__1::__function::__policy_storage const*) @ 0x8f0c143 in /usr/bin/clickhouse
2021.01.26 19:34:29.547736 [ 54 ] {} <Fatal> BaseDaemon: 6. ./obj-x86_64-linux-gnu/../src/Common/ZooKeeper/ZooKeeperNodeCache.cpp:0: zkutil::ZooKeeperNodeCache::get(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::function<void (Coordination::WatchResponse const&)>) @ 0x314ce5f4 in /usr/bin/clickhouse
2021.01.26 19:34:29.550517 [ 54 ] {} <Fatal> BaseDaemon: 7. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/functional:2191: zkutil::ZooKeeperNodeCache::get(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::shared_ptr<Poco::Event>) @ 0x314cd74a in /usr/bin/clickhouse
2021.01.26 19:34:29.554788 [ 54 ] {} <Fatal> BaseDaemon: 8. ./obj-x86_64-linux-gnu/../src/Common/Config/ConfigProcessor.cpp:373: Poco::XML::Node const* std::__1::__function::__policy_invoker<Poco::XML::Node const* (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)>::__call_impl<std::__1::__function::__default_alloc_func<DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&)::$_2, Poco::XML::Node const* (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)> >(std::__1::__function::__policy_storage const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) @ 0x31349875 in /usr/bin/clickhouse
2021.01.26 19:34:29.558804 [ 54 ] {} <Fatal> BaseDaemon: 9. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/functional:2221: DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&)::$_0::operator()(Poco::XML::Node const*, std::__1::function<Poco::XML::Node const* (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)> const&, char const*) const @ 0x3133218a in /usr/bin/clickhouse
2021.01.26 19:34:29.561962 [ 54 ] {} <Fatal> BaseDaemon: 10. ./obj-x86_64-linux-gnu/../src/Common/Config/ConfigProcessor.cpp:0: DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&) @ 0x3132e08c in /usr/bin/clickhouse
2021.01.26 19:34:29.565066 [ 54 ] {} <Fatal> BaseDaemon: 11. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:90: DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&) @ 0x3132f179 in /usr/bin/clickhouse
2021.01.26 19:34:29.568239 [ 54 ] {} <Fatal> BaseDaemon: 12. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:90: DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&) @ 0x3132f179 in /usr/bin/clickhouse
2021.01.26 19:34:29.571707 [ 54 ] {} <Fatal> BaseDaemon: 13. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:90: DB::ConfigProcessor::doIncludesRecursive(Poco::AutoPtr<Poco::XML::Document>, Poco::AutoPtr<Poco::XML::Document>, Poco::XML::Node*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&, std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >&) @ 0x3132f179 in /usr/bin/clickhouse
2021.01.26 19:34:29.575433 [ 54 ] {} <Fatal> BaseDaemon: 14. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:90: DB::ConfigProcessor::processConfig(bool*, zkutil::ZooKeeperNodeCache*, std::__1::shared_ptr<Poco::Event> const&) @ 0x3133c2ca in /usr/bin/clickhouse
2021.01.26 19:34:29.578976 [ 54 ] {} <Fatal> BaseDaemon: 15. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:118: DB::ConfigProcessor::loadConfigWithZooKeeperIncludes(zkutil::ZooKeeperNodeCache&, std::__1::shared_ptr<Poco::Event> const&, bool) @ 0x313423a2 in /usr/bin/clickhouse
2021.01.26 19:34:29.581804 [ 54 ] {} <Fatal> BaseDaemon: 16. ./obj-x86_64-linux-gnu/../contrib/poco/Foundation/include/Poco/AutoPtr.h:118: DB::ConfigReloader::reloadIfNewer(bool, bool, bool) @ 0x31359253 in /usr/bin/clickhouse
2021.01.26 19:34:29.584564 [ 54 ] {} <Fatal> BaseDaemon: 17. ./obj-x86_64-linux-gnu/../src/Common/Config/ConfigReloader.cpp:69: DB::ConfigReloader::run() @ 0x3135d18d in /usr/bin/clickhouse
2021.01.26 19:34:29.587440 [ 54 ] {} <Fatal> BaseDaemon: 18. ./obj-x86_64-linux-gnu/../src/Common/ThreadPool.h:179: ThreadFromGlobalPool::ThreadFromGlobalPool<void (DB::ConfigReloader::*)(), DB::ConfigReloader*>(void (DB::ConfigReloader::*&&)(), DB::ConfigReloader*&&)::'lambda'()::operator()() @ 0x31360594 in /usr/bin/clickhouse
2021.01.26 19:34:29.590497 [ 54 ] {} <Fatal> BaseDaemon: 19. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/functional:2090: void std::__1::__function::__policy_invoker<void ()>::__call_impl<std::__1::__function::__default_alloc_func<ThreadFromGlobalPool::ThreadFromGlobalPool<void (DB::ConfigReloader::*)(), DB::ConfigReloader*>(void (DB::ConfigReloader::*&&)(), DB::ConfigReloader*&&)::'lambda'(), void ()> >(std::__1::__function::__policy_storage const*) @ 0x31360379 in /usr/bin/clickhouse
2021.01.26 19:34:29.590795 [ 54 ] {} <Fatal> BaseDaemon: 20. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/functional:2210: ThreadPoolImpl<std::__1::thread>::worker(std::__1::__list_iterator<std::__1::thread, void*>) @ 0x9005977 in /usr/bin/clickhouse
2021.01.26 19:34:29.591557 [ 54 ] {} <Fatal> BaseDaemon: 21. ./obj-x86_64-linux-gnu/../contrib/libcxx/include/memory:1655: void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void ThreadPoolImpl<std::__1::thread>::scheduleImpl<void>(std::__1::function<void ()>, int, std::__1::optional<unsigned long>)::'lambda1'()> >(void*) @ 0x9013ff8 in /usr/bin/clickhouse
2021.01.26 19:34:29.591659 [ 54 ] {} <Fatal> BaseDaemon: 22. start_thread @ 0x9609 in /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
2021.01.26 19:34:29.591732 [ 54 ] {} <Fatal> BaseDaemon: 23. clone @ 0x122293 in /usr/lib/x86_64-linux-gnu/libc-2.31.so
2021.01.26 19:34:31.547215 [ 54 ] {} <Fatal> BaseDaemon: Calculated checksum of the binary: 73A029A06424D4E1E2081E2DE050B193. There is no information about the reference checksum.
```

#include <map>
#include <set>
#include <optional>
#include <memory>
#include <Poco/Mutex.h>
#include <Poco/UUID.h>
#include <Poco/Net/IPAddress.h>
#include <Poco/Util/Application.h>
#include <Common/Macros.h>
#include <Common/escapeForFileName.h>