[keyserver] Extract DB version functions to separate file

Summary:
In the following diff, I'm going to need to call these from setup-db.js. Without this diff, that change would result in a require cycle.

Depends on D4152

Test Plan: This is a simple refactor. I tested it in combination with stack (see test plan in D4151), and also used Flow

Reviewers: palys-swm, atul

Reviewed By: palys-swm, atul

Subscribers: Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4153