> (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.

delete_message_broker_schedule_pool = std::move(message_broker_schedule_pool);
            delete_access_control = std::move(access_control);
            
            trace_collector.reset();
            
            zookeeper.reset();
#if USE_ROCKSDB
            
            if (merge_tree_metadata_cache)
            {
                merge_tree_metadata_cache->shutdown();
                merge_tree_metadata_cache.reset();
            }
#endif
        }
        
        delete_system_logs.reset();
        delete_embedded_dictionaries.reset();
        delete_external_dictionaries_loader.reset();
        delete_external_user_defined_executable_functions_loader.reset();
        delete_user_defined_sql_objects_loader.reset();
        delete_ddl_worker.reset();
        delete_buffer_flush_schedule_pool.reset();
        delete_schedule_pool.reset();
        delete_distributed_schedule_pool.reset();
        delete_message_broker_schedule_pool.reset();
        delete_access_control.reset();
        total_memory_tracker.resetOvercommitTracker();
    }
    bool hasTraceCollector() const