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. 

{
    std::lock_guard lock(last_queue_update_exception_lock);
    return last_queue_update_exception;
}
void StorageReplicatedMergeTree::queueUpdatingTask()
{
    if (!queue_update_in_progress)
    {
        last_queue_update_start_time.store(time(nullptr));
        queue_update_in_progress = true;
    }
    try
    {
        queue.pullLogsToQueue(getZooKeeperAndAssertNotReadonly(), queue_updating_task->getWatchCallback(), ReplicatedMergeTreeQueue::UPDATE);
        last_queue_update_finish_time.store(time(nullptr));
        queue_update_in_progress = false;
    }
    catch (const Coordination::Exception & e)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        std::lock_guard lock(last_queue_update_exception_lock);
        last_queue_update_exception = getCurrentExceptionMessage(false);
        if (e.code == Coordination::Error::ZSESSIONEXPIRED)
        {
            restarting_thread.wakeup();
            return;
        }