> (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 <Storages/MarkCache.h>
#include <Storages/MergeTree/MergeList.h>
#include <Storages/MergeTree/MovesList.h>
#include <Storages/MergeTree/ReplicatedFetchList.h>
#include <Storages/MergeTree/MergeTreeData.h>
#include <Storages/MergeTree/MergeTreeSettings.h>
#include <Storages/CompressionCodecSelector.h>
#include <Storages/StorageS3Settings.h>
#include <Disks/DiskLocal.h>
#include <Disks/ObjectStorages/DiskObjectStorage.h>
#include <Disks/ObjectStorages/IObjectStorage.h>
#include <Disks/IO/ThreadPoolRemoteFSReader.h>
#include <Disks/IO/ThreadPoolReader.h>
#include <IO/SynchronousReader.h>
#include <TableFunctions/TableFunctionFactory.h>
#include <Interpreters/ActionLocksManager.h>
#include <Interpreters/ExternalLoaderXMLConfigRepository.h>
#include <Interpreters/TemporaryDataOnDisk.h>
#include <Core/Settings.h>
#include <Core/SettingsQuirks.h>
#include <Access/AccessControl.h>
#include <Access/ContextAccess.h>
#include <Access/EnabledRolesInfo.h>
#include <Access/EnabledRowPolicies.h>
#include <Access/QuotaUsage.h>
#include <Access/User.h>
#include <Access/Credentials.h>
#include <Access/SettingsProfile.h>
#include <Access/SettingsProfilesInfo.h>
#include <Access/SettingsConstraintsAndProfileIDs.h>