[lib] generate message store ops for threads while merging messages

Summary:
This code generates operations needed for updating threads part of message store. Currently, only for comparing results, but after some time we can rely only on generated ops.

Depends on D7397

Test Plan:

1. Test app (multiple flows) and check if assert does not fail
2. Comment some ops code and make sure assert will fail (to check if asserting it works as supposed to)

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7398