[Reviewer: Rob][From: Alex] Cope with timers that have already popped more gracefully. 

Rob, please can you review this fix for issue #37? Analysis of the problem is in the issue. 

Note that I've recently rewritten the timer store, so it might have changed since you last saw it. Grab me if you have any questions. 

Tested in UT. My ralf load testing should confirm whether it fixes the issue live.