> (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 (!backup_settings.internal)
        {
            backup->finalizeWriting();
            num_files = backup->getNumFiles();
            uncompressed_size = backup->getUncompressedSize();
            compressed_size = backup->getCompressedSize();
        }
        
        backup.reset();
        LOG_INFO(log, "{} {} was created successfully", (backup_settings.internal ? "Internal backup" : "Backup"), backup_name_for_logging);
        setStatus(backup_id, BackupStatus::BACKUP_CREATED);
        setNumFilesAndSize(backup_id, num_files, uncompressed_size, compressed_size);
    }
    catch (...)
    {
        
        if (called_async)
        {
            tryLogCurrentException(log, fmt::format("Failed to make {} {}", (backup_settings.internal ? "internal backup" : "backup"), backup_name_for_logging));
            setStatusSafe(backup_id, BackupStatus::BACKUP_FAILED);
            sendCurrentExceptionToCoordination(backup_coordination, backup_settings.host_id);
        }
        else
        {
            
            throw;
        }
    }
}
OperationID BackupsWorker::startRestoring(const ASTPtr & query, ContextMutablePtr context)