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 String & query,
    ContextMutablePtr context,
    const String & database,
    const String & file_name,
    bool create,
    bool has_force_restore_data_flag)
{
    ParserCreateQuery parser;
    ASTPtr ast = parseQuery(
        parser, query.data(), query.data() + query.size(), "in file " + file_name, 0, context->getSettingsRef().max_parser_depth);
    auto & ast_create_query = ast->as<ASTCreateQuery &>();
    ast_create_query.setDatabase(database);
    InterpreterCreateQuery interpreter(ast, context);
    interpreter.setInternal(true);
    if (!create)
    {
        interpreter.setForceAttach(true);
        interpreter.setForceRestoreData(has_force_restore_data_flag);
    }
    interpreter.setLoadDatabaseWithoutTables(true);
    interpreter.execute();
}
static bool isSystemOrInformationSchema(const String & database_name)
{
    return database_name == DatabaseCatalog::SYSTEM_DATABASE ||
           database_name == DatabaseCatalog::INFORMATION_SCHEMA ||
           database_name == DatabaseCatalog::INFORMATION_SCHEMA_UPPERCASE;
}
static void loadDatabase(