[lib] Return mergeNewMessages(...) directly where possible in reduceMessageStore(...)

Summary:
Basically D4337, but for all the other mergeNewMessages(...) callsites.

This is just a noop refactor, trying to simplify message-reducer

Depends on D4337

Test Plan: Close reading, flow

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4338