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. 

{
    auto zookeeper = getZooKeeper();
    Coordination::Stat stat;
    String s = zookeeper->get(zookeeper_path + "/pinned_part_uuids", &stat);
    std::lock_guard lock(pinned_part_uuids_mutex);
    
    if (pinned_part_uuids->stat.version < stat.version)
    {
        auto new_pinned_part_uuids = std::make_shared<PinnedPartUUIDs>();
        new_pinned_part_uuids->fromString(s);
        new_pinned_part_uuids->stat = stat;
        pinned_part_uuids = new_pinned_part_uuids;
    }
}
void StorageReplicatedMergeTree::checkPartChecksumsAndAddCommitOps(const zkutil::ZooKeeperPtr & zookeeper,