[lib] Fix sidebar source clustering bug

Summary: To reproduce the bug, create sidebar source message which references some message from at least 5 mins ago and another message, younger than sidebar source. Open a thread and notice that the messages were displayed as one cluster with the date of initial message, even though younger message should not be included.

Test Plan: Use reproduction steps and notice that the messages are displayed as two separate clusters.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D605