Support sqlite/postgresql backend

[Glandos]

In the Python version of syncserver, SQL was handled by SQLAlchemy, and thus every major SQL backends were available. I personally use sqlite because I'm nearly a single user.

As a I discover this Rust rewrite (:clap:), I see that only MySQL and Spanner are currently supported. This is a bit unfortunate, since it lefts all sqlite and postgresql existing users without alternative. I withdraw MySQL very long time ago, to have only one DB engine to manage.

I also understand that in can be a non-goal to support other architecture than Mozilla. I just wanted to ask: what are the plans for backends?

[jrconlin]

I don't think any of us on the 3 person team are opposed to supporting other databases, it's just that we've got a fair bit on our plate right now and are focused on the spanner side of things at the moment.

The good news is that the SQL calls we do are very simple. We do need to make sure that the MySQL calls are compatible with what we're doing with Spanner, but that should be more of a function check.

Needless to say, we always welcome folks helping us out so we can deliver this feature sooner.

[fzzzy]

@Glandos We use diesel for the mysql backend. Diesel supports mysql, postgres, and sqlite. So perhaps with a little tinkering the existing mysql diesel code could be repurposed. I'll mark it as a good first bug.

[somini]

#935 implements this, any reason why this wasn't merged?