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. 

{
    String path = context->getPath() + "metadata/" + database_name;
    String metadata_file = path + ".sql";
    if (fs::exists(fs::path(metadata_file)))
    {
        
        loadDatabase(context, database_name, path, true);
    }
    else
    {
        checkUnsupportedVersion(context, database_name);
        
        String database_create_query = "CREATE DATABASE ";
        database_create_query += database_name;
        database_create_query += " ENGINE=";
        database_create_query += default_engine;
        executeCreateQuery(database_create_query, context, database_name, "<no file>", true, true);
    }
}
static void convertOrdinaryDatabaseToAtomic(ContextMutablePtr context, const DatabasePtr & database)