Provide job timeouts

[nfachan]

Clients will want a way to set timeouts on jobs. The most obvious use case is for test frameworks, but it's a generally useful feature.

We could implement this using per-job tracking (#64) and per-job cancelation (#63), but implementing those two features just to support timeouts seems like a more work than is necessary. Also, it would put the burden on the client, meaning we'd have to re-implement cancelation in every client library (in multiple languages, for example). Plus, it wouldn't be the simplest thing to implement on the client since a job may be started, the maybe the worker goes away, and the job gets restarted on a different worker, and then maybe the job gets pre-empted, etc. The client would have to be pretty sophisticated about tracking the various states the job could be in.

I imagine this would be an Option<Duration> in the JobDetails. We'd probably add another variant to JobStatus for TimedOut, or we'd do something equivalent. The worker would be responsible for implementing the timeout. It would start the timer ticking when it actually started executing the job.