[lib] Break ties in message timestamps with message IDs

Summary: I ran a script that generated multiple messages with the same timestamp. message-reducer doesn't know how to break these ties, but when it uses the lower message ID, activityUpdater rejects it because it sees a newer message. We need to make sure message-reducer breaks ties in the same way that activityUpdater does.

Test Plan: loaded website and make sure it didn't crash

Reviewers: atul, palys-swm

Subscribers: KatPo, zrebcu411, Adrian, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D805