Replaced the record_activity mechanism of indicating aliveness with a heartbeat running on its own thread
Tidied up the implementation of the heartbeat thread
Add a test that repeated heartbeats push out the TTL on the worker activity key
Wrap up starting the heartbeat thread and prevent any race conditions
Remove old activity_timeout construction arg.
Daemonize the heartbeat thread so it doesn't prolong the suffering
Merge pull request #45 from gamechanger/tl-heartbeat

Replace brittle timeout with a background heartbeat thread