Add implicit GROUP BY sorting when compatibility target is 5.7

## Enhancement

In MySQL 5.7, there is a (deprecated) feature: GROUP BY implies sorting.

In MySQL 8.0, this feature is removed, because implicit sorting prevents more optimal plans.

Some MySQL applications like Wordpress relies on this feature to work correctly. Thus, we could leave the choice to users: Compatibility or performance.

Refs:

- http://www.tocker.ca/an-update-on-group-by-implicit-sort.html
- http://www.tocker.ca/2013/10/21/heads-up-implicit-sorting-by-group-by-is-deprecated-in-mysql-5-6.html
- https://dev.mysql.com/blog-archive/removal-of-implicit-and-explicit-sorting-for-group-by/
- [Wordpress Compatibility matrix](https://make.wordpress.org/hosting/handbook/compatibility/#wordpress-php-mysql-mariadb-versions)