> (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 (shutdown_called)
            return;
        shutdown_called = true;
        
        
        
        if (external_dictionaries_loader)
            external_dictionaries_loader->enablePeriodicUpdates(false);
        if (external_user_defined_executable_functions_loader)
            external_user_defined_executable_functions_loader->enablePeriodicUpdates(false);
        if (user_defined_sql_objects_loader)
            user_defined_sql_objects_loader->stopWatching();
        Session::shutdownNamedSessions();
        
        if (backups_worker)
            backups_worker->shutdown();
        /**  After system_logs have been shut down it is guaranteed that no system table gets created or written to.
          *  Note that part changes at shutdown won't be logged to part log.
          */
        if (system_logs)
            system_logs->shutdown();
        DatabaseCatalog::shutdown();