[lib] refactor code to update reply count and unread status to function

Summary:
ENG-9273.

I need to reuse this function to bump reply count as part of useSendComposableDMOperation.

Did some additional cleanup to avoid selecting the same data multiple times.

Test Plan: Flow, functionality tested later in the stack

Reviewers: tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13426