**Describe the issue**
ClickHouse is unable to start up after upgrading from 20.8 to 21.8. 

There is a replicated table with `:` in zk path and it couldn't be loaded with:

`2021.09.15 03:49:58.435071 [ 633075 ] {} <Error> Application: DB::Exception: Unknown auxiliary ZooKeeper name '/test_prefix'. If it's required it can be added to the section <auxiliary_zookeepers> in config.xml: Cannot attach table `db1`.`table1` from metadata file /var/lib/clickhouse/metadata/db1/table1.sql from query ATTACH TABLE db1.table1 (`field1` String, `date` Date) ENGINE = ReplicatedMergeTree('/test_prefix:db1/{shard}/table1', '{replica}') PARTITION BY toMonth(date) PRIMARY KEY field1 ORDER BY field1 SETTINGS index_granularity = 8192: while loading database `db1` from path /var/lib/clickhouse/metadata/db1
`

**Expected behavior**
ClickHouse server is able to startup after upgrade without schema change. "test_prefix" in "/test_prefix:db1/{shard}/table1" is not interpreted as an auxiliary ZooKeeper name.

{
    auto it = local_scalars.find(name);
    if (local_scalars.end() == it)
        return nullptr;
    return &it->second;
}
Tables Context::getExternalTables() const
{
    assert(!isGlobalContext() || getApplicationType() == ApplicationType::LOCAL);
    auto lock = getLock();
    Tables res;
    for (const auto & table : external_tables_mapping)
        res[table.first] = table.second->getTable();
    auto query_context_ptr = query_context.lock();
    auto session_context_ptr = session_context.lock();
    if (query_context_ptr && query_context_ptr.get() != this)
    {
        Tables buf = query_context_ptr->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    else if (session_context_ptr && session_context_ptr.get() != this)
    {
        Tables buf = session_context_ptr->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    return res;
}
void Context::addExternalTable(const String & table_name, TemporaryTableHolder && temporary_table)