[lib] Check the timestamps when joining threads

Summary:
Only update the membership when the operation is younger than the saved timestamp.

https://linear.app/comm/issue/ENG-9115/update-jointhreadspec

Depends on D13216

Test Plan: Created two join thread operations and made sure the timestamp is set correctly.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13217