[lib] move calling getThreadUpdatesForNewMessages to reduceThreadInfos

Summary:
This is a fix for ENG-9453.

Alternative is introducing another queue in useSendComposableDMOperation to make sure we're always calculating replies count on the newest thread info, but I prefer this solution and it's cleaner.

Test Plan: Send a lot of messages and checking replies count - did test multiple times as there is no single way to reproduce it.

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13661