Reviewer Ellie - Fix use after delete

This fixes the issues we've been discussing with time shifting by small millisecond amounts.  Fix is to always round time down to the closest 10ms before calculating buckets, that way any small shifts in time are ignored.

This fixes https://github.com/Metaswitch/chronos/issues/19.

I've added UTs for short and mid timers (to make sure we also cover the case where we move to close to a 1 second shift).

I want to get this into Elite, so if you could review early on Monday that would be appreciated!