[lib] refactor keyserverStoreOpsHandlers to process ops on entire KeyserverStore

Summary:
Context here: https://phab.comm.dev/D10934#inline-66526.
This was also suggested here: https://phab.comm.dev/D10755#inline-65783

Test Plan: Flow - this is just a refactor

Reviewers: inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10966