[lib] refactor processing server requests in UserStore to ops

Summary:
Convert logic for ops approach.

Depends on D9799

Test Plan:

1. Update sessionCheckFrequency to lower value
2. Update some users directly in database
3. Delete some users from database

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9800