[keyserver] Introduce assertValidDBType

Summary: This diff is a simple refactor, and should be a no-op. In the next diff I'll modify assertValidDBType to deprecate MySQL.

Test Plan:

1. Flow
2. I ran keyserver locally and made sure things generally still worked

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4832