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 (it->is_symlink())
            continue;
        if (it->is_directory())
            continue;
        const auto current_file = it->path().filename().string();
        
        if (fs::path(current_file).extension() == ".sql")
        {
            String db_name = fs::path(current_file).stem();
            if (!isSystemOrInformationSchema(db_name))
                databases.emplace(unescapeForFileName(db_name), fs::path(path) / db_name);
        }
        
        if (fs::path(current_file).extension() == ".tmp")
        {
            LOG_WARNING(log, "Removing temporary file {}", it->path().string());
            try
            {
                fs::remove(it->path());
            }
            catch (...)
            {
                
                tryLogCurrentException(log);
            }
        }
    }