> (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.

BackupSettings backup_settings,
        std::shared_ptr<IBackupCoordination> backup_coordination,
        const ContextPtr & context,
        ContextMutablePtr mutable_context,
        bool called_async);
    OperationID startRestoring(const ASTPtr & query, ContextMutablePtr context);
    void doRestore(
        const std::shared_ptr<ASTBackupQuery> & restore_query,
        const OperationID & restore_id,
        const String & backup_name_for_logging,
        const BackupInfo & backup_info,
        RestoreSettings restore_settings,
        std::shared_ptr<IRestoreCoordination> restore_coordination,
        ContextMutablePtr context,
        bool called_async);
    void addInfo(const OperationID & id, const String & name, bool internal, BackupStatus status);
    void setStatus(const OperationID & id, BackupStatus status, bool throw_if_error = true);
    void setStatusSafe(const String & id, BackupStatus status) { setStatus(id, status, false); }
    void setNumFilesAndSize(const OperationID & id, size_t num_files, UInt64 uncompressed_size, UInt64 compressed_size);
    ThreadPool backups_thread_pool;
    ThreadPool restores_thread_pool;
    std::unordered_map<OperationID, Info> infos;
    std::condition_variable status_changed;
    std::atomic<size_t> num_active_backups = 0;
    std::atomic<size_t> num_active_restores = 0;
    mutable std::mutex infos_mutex;
    Poco::Logger * log;
};
}