broker needs to intervene if a runner times out

[ckerr]

Currently, if a runner disappears in the middle of a job, that job will just hang indefinitely.

The broker should notice when a runner isn't communicating. It could definitely amend the log, and it perhaps should also mark the job as unclaimed so that another runner can try.

[ckerr]

This is still a real issue. We should probably add a timeout in the broker to mark a claimed task as unclaimed if it sits idle for more than a couple of minutes.

Now, if there's just something really wrong with a fiddle or with a runner, we could get into an endless loop of [ runner claims -> runner crashes -> runner is restarted -> broker unclaims ]. We may want to consider a circuit breaker of some sort to ensure this pathological case doesn't run more than a few loops.

Related to #176.