Correct bug with really fast polling servers and slow workers.

When the server was under really high load from clients, and the worker
had a slow heartbeat rate, it was possible to create false positive worker
expiring, which would cause jobs to fail.

Change-Id: I09c969b3a8833d85202b8c606aa6281deee3518d
Merge pull request #65 from robertmaynard/incorrect_heartbeat_tracking

Correct bug with really fast polling servers and slow workers.