Better support for returnimg statement on back ends

[marlon-sousa]

I am investigating how to better integrate returning statements to back ends which support them.

MariaDB above version 10 and latest sqlite versions already offer this support, and a better handling of how returning statement works will benefit almost everyone using inserts, multiple multiple and updates, as well as some use cases for delete.

A reference implementation is at pr #155

This issue is a required work to address SeaQL/sea-orm#183

Technical implementation

For details, please see rreference implementation #155