[lib] Move message update reducer into specs

Summary:
Use a similar approach to the original one, which is based on mutating the result while iterating the array.

Depends on D9318

https://linear.app/comm/issue/ENG-4241/handle-processupdatesactiontype-as-a-part-of-a-spec

Test Plan: Check if the new code produces the same result os the old one.

Reviewers: kamil, bartek, inka

Reviewed By: kamil

Subscribers: ashoat, michal

Differential Revision: https://phab.comm.dev/D9319