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 zkutil::ZooKeeperPtr & zookeeper, const String & zookeeper_node, int32_t mode,
    bool replace_existing_lock, const String & path_to_set_hardlinked_files, const NameSet & hardlinked_files)
{
    
    
    bool created = false;
    for (int attempts = 5; attempts > 0; --attempts)
    {
        try
        {
            
            
            
            if (mode == zkutil::CreateMode::Persistent)
                zookeeper->createAncestors(zookeeper_node);
            if (replace_existing_lock && zookeeper->exists(zookeeper_node))
            {
                Coordination::Requests ops;
                ops.emplace_back(zkutil::makeRemoveRequest(zookeeper_node, -1));
                ops.emplace_back(zkutil::makeCreateRequest(zookeeper_node, "", mode));