[lib] Add function for finding the time of the newest message per keyserver

Summary:
issue: https://linear.app/comm/issue/ENG-5723/update-mergenewmessages-function
I need a function that for an array of messages will give me the time of the newest message for every keyserver

Test Plan: Ran the provided test

Reviewers: michal, kamil, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9881