> (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 <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>
#include <Common/EventNotifier.h>
#include <Common/setThreadName.h>
#include <Common/Stopwatch.h>
#include <Common/formatReadable.h>
#include <Common/Throttler.h>
#include <Common/thread_local_rng.h>
#include <Common/FieldVisitorToString.h>
#include <Common/getMultipleKeysFromConfig.h>
#include <Coordination/KeeperDispatcher.h>
#include <Compression/ICompressionCodec.h>
#include <Core/BackgroundSchedulePool.h>
#include <Formats/FormatFactory.h>
#include <Databases/IDatabase.h>
#include <Storages/IStorage.h>