[lib] Check the timestamps when adding the viewer to thread members

Summary:
Check the timestamp and update the membership only when necessary.

https://linear.app/comm/issue/ENG-9114/update-addviewertothreadmembersspec

Depends on D13217

Test Plan: Created two add_viewer_to_thread_members operations and checked if the resulting timestamps are correct (all the timestamps should equal the first operation time except the membership.role timestamp).

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13218