[lib] Extract read status operations logic

Summary:
Compute the state in a single place.

https://linear.app/comm/issue/ENG-8930/unify-the-approach-to-replies-count-and-unread-status-updates

Depends on D12975

Test Plan: Processed an operation representing creation of a new thread with a viewer and a new user, created by that user. Checked if the thread was created and was set to unread.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12987