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. 

{
        /** If we can not download the part we need for some merge, it's better not to try to get other parts for this merge,
          * but try to get already merged part. To do this, move the action to get the remaining parts
          * for this merge at the end of the queue.
          */
        try
        {
            auto parts_for_merge = queue.moveSiblingPartsForMergeToEndOfQueue(entry.new_part_name);
            if (!parts_for_merge.empty() && replica.empty())
            {
                LOG_INFO(log, "No active replica has part {}. Will fetch merged part instead.", entry.new_part_name);
                
                enqueuePartForCheck(entry.new_part_name);
                return false;
            }
            /** If no active replica has a part, and there is no merge in the queue with its participation,
              * check to see if any (active or inactive) replica has such a part or covering it.
              */
            if (replica.empty())
                enqueuePartForCheck(entry.new_part_name);
        }
        catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
        }
        throw;
    }
    return true;
}
DataPartStoragePtr StorageReplicatedMergeTree::executeFetchShared(