[keyserver] refactor functions to check rotate/forget prekeys

Summary:
Issue: ENG-3944.

This is part of D8476 (breaking this for smaller diffs because it's complex).
Addresses:

• https://phab.comm.dev/D8476#inline-58274
• https://phab.comm.dev/D8476#inline-58273

Fixes:

• https://linear.app/comm/issue/ENG-5197/fix-last-prekey-publish-time-usage

Test Plan:

• Flow
• check if time last publish time is properly retrieved

Reviewers: varun, marcin, ashoat

Reviewed By: marcin, ashoat

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9419