Update the persistent timestamp even if there are no oplog entries

We use the oplog timestamp recorded in the db as an indicator of whether our
oplog tailers are up to date or not. However, on the rare occasion when we
have a totally idle database, the timestamps don't get updated.

This change ensures that the timestamp is updated, even when new entries
aren't being written to the oplog. As long as we grab the timestamp before
querying the oplog, if we then query the oplog and get no results, we know
there won't be any entries older than the current server timestamp.