[libcommhistory] Finish 'Always sort events by endTime'

Commit 75d4ea36 gave reasoning for sorting events by endTime instead
of startTime (namely, for messages the startTime is the sender's time
tag and we want the time it was received, which is the endTime)

However this change was not made in the GroupManager, causing the
test for this new behavior to fail. (GroupModelTest::endTimeUpdate)