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. 

{
    {
        std::lock_guard lock(existing_nodes_cache_mutex);
        if (existing_nodes_cache.contains(path))
            return true;
    }
    bool res = getZooKeeper()->exists(path);
    if (res)
    {
        std::lock_guard lock(existing_nodes_cache_mutex);
        existing_nodes_cache.insert(path);
    }
    return res;
}
std::optional<EphemeralLockInZooKeeper>