> (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.

if (merge_mutate_executor)
            merge_mutate_executor->wait();
        if (fetch_executor)
            fetch_executor->wait();
        if (moves_executor)
            moves_executor->wait();
        if (common_executor)
            common_executor->wait();
        TransactionLog::shutdownIfAny();
        std::unique_ptr<SystemLogs> delete_system_logs;
        std::unique_ptr<EmbeddedDictionaries> delete_embedded_dictionaries;
        std::unique_ptr<ExternalDictionariesLoader> delete_external_dictionaries_loader;
        std::unique_ptr<ExternalUserDefinedExecutableFunctionsLoader> delete_external_user_defined_executable_functions_loader;
        std::unique_ptr<IUserDefinedSQLObjectsLoader> delete_user_defined_sql_objects_loader;
        std::unique_ptr<BackgroundSchedulePool> delete_buffer_flush_schedule_pool;
        std::unique_ptr<BackgroundSchedulePool> delete_schedule_pool;
        std::unique_ptr<BackgroundSchedulePool> delete_distributed_schedule_pool;
        std::unique_ptr<BackgroundSchedulePool> delete_message_broker_schedule_pool;
        std::unique_ptr<DDLWorker> delete_ddl_worker;
        std::unique_ptr<AccessControl> delete_access_control;
        
        
        {
            auto lock = std::lock_guard(mutex);
            delete_ddl_worker = std::move(ddl_worker);
        }
        
        
        delete_ddl_worker.reset();
        {