> (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 <Backups/IBackupEntry.h>
#include <Backups/BackupEntriesCollector.h>
#include <Backups/BackupCoordinationStage.h>
#include <Backups/BackupCoordinationRemote.h>
#include <Backups/BackupCoordinationLocal.h>
#include <Backups/RestoreCoordinationRemote.h>
#include <Backups/RestoreCoordinationLocal.h>
#include <Backups/RestoreSettings.h>
#include <Backups/RestorerFromBackup.h>
#include <Interpreters/Cluster.h>
#include <Interpreters/Context.h>
#include <Interpreters/executeDDLQueryOnCluster.h>
#include <Parsers/ASTBackupQuery.h>
#include <Parsers/ASTFunction.h>
#include <Common/Exception.h>
#include <Common/Macros.h>
#include <Common/logger_useful.h>
#include <Common/setThreadName.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int BAD_ARGUMENTS;
    extern const int LOGICAL_ERROR;
}
using OperationID = BackupsWorker::OperationID;
namespace Stage = BackupCoordinationStage;
namespace
{
    std::shared_ptr<IBackupCoordination> makeBackupCoordination(const String & coordination_zk_path, const ContextPtr & context, bool is_internal_backup)