[lib] Check the timestamps when leaving a thread

Summary:
Only remove the editor when the operation is more recent than the timestamp.

https://linear.app/comm/issue/ENG-9116/update-leavethreadspec

Depends on D13219

Test Plan: Create a thread, add some members, and remove some. Check if the timestamps behave correctly, and the membership is correct.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13220