Fix Infinite looping when shotgun drops events

[jcytryn]

We're finding that our instance of shotgun will often skip many event ids in the event log. However, the shotgunEvents daemon currently makes the assumption that a drop just indicates inactivity in the daemon. The result is each time a dropped id exists within a batch of batch_size (e.g. 500), it will infinitely loop over those 500 events until the 5 minute timeout happens. This should be fixed to do some sort of check against shotgun such that older events (beyond some threshold) should not be added to the backlog.