Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

queue_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        std::lock_guard lock(last_queue_update_exception_lock);
        last_queue_update_exception = getCurrentExceptionMessage(false);
        queue_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
}
void StorageReplicatedMergeTree::mutationsUpdatingTask()
{
    try
    {
        queue.updateMutations(getZooKeeper(), mutations_updating_task->getWatchCallback());
    }
    catch (const Coordination::Exception & e)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        if (e.code == Coordination::Error::ZSESSIONEXPIRED)
            return;
        mutations_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        mutations_updating_task->scheduleAfter(QUEUE_UPDATE_ERROR_SLEEP_MS);
    }
}
ReplicatedMergeTreeQueue::SelectedEntryPtr StorageReplicatedMergeTree::selectQueueEntry()