[Future] Move Database Backends to a "backends/" directory

[deavmi]

Just putting this here for the future to further modularise it all.

I assume only the abstraction library will need updating to the new pathnames for the current databases we support.

[cdpxe]

Hi, sorry, but its too few files; I would keep that all in src/. Changing directories can negatively influence stability/inner-workings of the package.

[deavmi]

Alright. Will close.