[lib] Merge identical action handlers in message-reducer

Summary:
Saw that the handlers for each of these action types was identical, decided to merge them to reduce unnecessary repetition.

Depends on D4339

Test Plan: Close reading, will write some unit tests

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4340