> (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.

context_in_use,
                        mutable_context,
                         true);
                });
        }
        else
        {
            doBackup(
                backup_query,
                backup_id,
                backup_name_for_logging,
                backup_info,
                backup_settings,
                backup_coordination,
                context_in_use,
                mutable_context,
                 false);
        }
        return backup_id;
    }
    catch (...)
    {
        tryLogCurrentException(log, fmt::format("Failed to start {} {}", (backup_settings.internal ? "internal backup" : "backup"), backup_name_for_logging));
        
        setStatusSafe(backup_id, BackupStatus::BACKUP_FAILED);
        sendCurrentExceptionToCoordination(backup_coordination, backup_settings.host_id);
        throw;
    }
}
void BackupsWorker::doBackup(