[lib] Rename locallyUnique... to pending... to improve readability

Summary:
Follow-up after D4443. The diff reverts renaming pending to locallyUnique made a few months ago.
Linear: https://linear.app/comm/issue/ENG-1374/rename-locallyuniquethreadid-to-pendingthreadid

Test Plan: Web and mobile apps build correctly and work as before. Confirmed, that features using locallyUnique/pending IDs still works correctly - e.g. thread creation and storing drafts for pending threads.

Reviewers: tomek, atul

Reviewed By: atul

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4644