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 (disk->supportZeroCopyReplication())
    {
        FreezeMetaData meta;
        meta.fill(*this);
        meta.save(disk, backup_part_path);
    }
}
void StorageReplicatedMergeTree::adjustCreateQueryForBackup(ASTPtr & create_query) const