1. After a period of wait (where last_master_timestamp=0)
   do NOT restore the last_master_timestamp to the timestamp
   of the last executed event (which would mean we've just
   executed it, and we're that much behind the master).

2. Update last_master_timestamp before executing the event,
   not after.

Take the approach from the this commit (but with a different test
case that actually makes sense):

commit 0c75ab453fb8c5439576af8fe5add7a1b89f1569
Author: Luis Soares <luis.soares@sun.com>
Date:   Thu Apr 15 17:39:31 2010 +0100

    BUG#52166: Seconds_Behind_Master spikes after long idle period