> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

#include <Parsers/ASTIdentifier.h>
#include <Common/StackTrace.h>
#include <Common/Config/ConfigHelper.h>
#include <Common/Config/ConfigProcessor.h>
#include <Common/Config/AbstractConfigurationComparison.h>
#include <Common/ZooKeeper/ZooKeeper.h>
#include <Common/ShellCommand.h>
#include <Common/logger_useful.h>
#include <base/EnumReflection.h>
#include <Common/RemoteHostFilter.h>
#include <Interpreters/AsynchronousInsertQueue.h>
#include <Interpreters/DatabaseCatalog.h>
#include <Interpreters/JIT/CompiledExpressionCache.h>
#include <Storages/MergeTree/BackgroundJobsAssignee.h>
#include <Storages/MergeTree/MergeTreeBackgroundExecutor.h>
#include <Storages/MergeTree/MergeTreeDataPartUUID.h>
#include <Storages/MergeTree/MergeTreeMetadataCache.h>
#include <Interpreters/SynonymsExtensions.h>
#include <Interpreters/Lemmatizers.h>
#include <Interpreters/ClusterDiscovery.h>
#include <Interpreters/TransactionLog.h>
#include <Interpreters/Cache/FileCacheFactory.h>
#include <filesystem>
#include <re2/re2.h>
#include <Storages/StorageView.h>
#include <Parsers/ASTFunction.h>
#include <base/find_symbols.h>
#include <Interpreters/Cache/FileCache.h>
#if USE_ROCKSDB
#include <rocksdb/table.h>