[lib] Rename threadsToMessageIDsFromMessageInfos to mapThreadsToMessageIDsFromOrderedMessageInfos

Summary:
I think this is a better, more descriptive name. I think it's good to include the fact that the messageInfos are ordered in the name so it's more evident at the callsite.

This is an "opinionated" change, so open to abandoning if others disagree.

Test Plan: flow

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4334