[lib] generate message store ops for threads while starting to send a message

Summary:
This code generates operations needed for updating threads part of message store after starting to send a message.

In this case it's hard to early exist and remove indentation and have one place for processing ops and asserting results.

Depends on D7412

Test Plan: Try to send message using different flows on both web and native

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7417