[lib] generate message store ops for threads after pruning messages

Summary:
This code generates operations needed for updating threads part of message store after pruning messages.

Depends on D7419

Test Plan: Force app to prune messages (eg. modify msInHour) and check asserting

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7420