[keyserver] Extract "db_version" metadata key

Summary:
This is a simple change so we avoid redefining this string.

Depends on D4153

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, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4154