[lib] Add timestamp check when flipping unread status

Summary:
This diff fixes messages appearing unread by adding a timestamp check when getting new messages.

https://linear.app/comm/issue/ENG-9727/thick-threads-appearing-unread

Test Plan:

1. User A logs to web and mobile
2. User B logs to web
3. User's A mobile is in background
4. User B sends DM to A
5. User A reads the message on web
6. User A brings mobile back to foreground. Verify the last message is not unread.
7. Repeat a few times

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13766