> (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.

params.only_replica_num = backup_settings.replica_num;
            params.access_to_check = required_access;
            backup_settings.copySettingsToQuery(*backup_query);
            
            mutable_context->setSetting("distributed_ddl_task_timeout", Field{0});
            mutable_context->setSetting("distributed_ddl_output_mode", Field{"none"});
            executeDDLQueryOnCluster(backup_query, mutable_context, params);
            
            auto all_hosts = BackupSettings::Util::filterHostIDs(
                backup_settings.cluster_host_ids, backup_settings.shard_num, backup_settings.replica_num);
            backup_coordination->waitForStage(all_hosts, Stage::COMPLETED);
        }
        else
        {
            backup_query->setCurrentDatabase(context->getCurrentDatabase());
            
            BackupEntries backup_entries;
            {
                BackupEntriesCollector backup_entries_collector{backup_query->elements, backup_settings, backup_coordination, context};
                backup_entries = backup_entries_collector.run();
            }
            
            writeBackupEntries(backup, std::move(backup_entries), backups_thread_pool);
            
            backup_coordination->setStage(backup_settings.host_id, Stage::COMPLETED, "");
        }
        size_t num_files = 0;
        UInt64 uncompressed_size = 0;
        UInt64 compressed_size = 0;