[identity] Add function to batch-get device lists

Summary:
Part of GetDeviceListsForUsers RPC rewrite. Details in ENG-8843.

Added a function that fetches latest device lists by using the timestamp attribute from users table. This way, we can get primary keys for device lists
without using Query, and thus can do BatchGet

Test Plan: Detailed test plan in ENG-8843.

Reviewers: varun, will, kamil

Reviewed By: varun, will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12888