Change heap ordering to use heap size as tiebreaker.

The idea is to preserve FIFO, so first in wins the tiebreak. Current behavior relies on memory addresses, which may or may not be increasing with each new entry on systems with large amounts of RAM, but systems with less RAM will need to GC more often, leading to situations where newer Timeouts may have lower memory addresses.  https://docs.python.org/2/library/heapq.html#priority-queue-implementation-notes suggests using an entry count as a tiebreaker.