[lib] refactor processing updates in UserStore to ops

Summary:
Convert logic for ops approach.

Depends on D9800

Test Plan:

1. Login to two accounts that have friend relationship
2. On second delete account
3. On first make sure update is properly processed

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9801