Fix tests which were broken by incomplete changes.

1. Switch from utcnow_ts to utcnow in mongo driver;
2. Add the strict expiration filtering back.

It makes sense to relax the TTL restriction on messages, but in
the current version, get msg & list msgs behavior differently.
So at this point of time, this 'feature' should be regarded as a
bug, and the tests should be fixed before we land this 'feature'.

Change-Id: I3606b9d6459ab6cfc91b431fa66acdbaa2874e1e