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. 

const DataPartPtr & part, Coordination::Requests & ops, String part_name, NameSet * absent_replicas_paths)
{
    if (part_name.empty())
        part_name = part->name;
    auto local_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksums(
        part->getColumns(), part->checksums);
    Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    std::shuffle(replicas.begin(), replicas.end(), thread_local_rng);
    bool has_been_already_added = false;
    for (const String & replica : replicas)
    {
        String current_part_path = fs::path(zookeeper_path) / "replicas" / replica / "parts" / part_name;
        String part_zk_str;
        if (!zookeeper->tryGet(current_part_path, part_zk_str))
        {
            if (absent_replicas_paths)
                absent_replicas_paths->emplace(current_part_path);
            continue;
        }
        ReplicatedMergeTreePartHeader replica_part_header;
        if (part_zk_str.empty())
        {
            String columns_str;
            String checksums_str;
            if (zookeeper->tryGet(fs::path(current_part_path) / "columns", columns_str) &&
                zookeeper->tryGet(fs::path(current_part_path) / "checksums", checksums_str))
            {
                replica_part_header = ReplicatedMergeTreePartHeader::fromColumnsAndChecksumsZNodes(columns_str, checksums_str);
            }
            else