Add Graceful Shutdown Hook

[luke-rucker]

Since the worker traps shutdown signals to the process so it can gracefully handle in-progress jobs, this makes it impossible for consumers of the library to perform any kind of graceful shutdown of their own.

This PR adds the ability for a cleanup function to be passed to the worker to be called after in-progress jobs are completed.

Let me know what you think!

[jbielick]

This looks pretty good. I'm curious, would a worker.emit('stop') or something do the trick? Any reason we need to await the shutdown task?

[luke-rucker]

My thought with awaiting the shutdown task was so that the task would respect the shutdownTimeout of the worker. Although, one could argue that this is mixing concerns and the shutdownTimeout should only affect in-progress jobs, not the graceful shutdown of the whole process.

I'm more than happy to modify the PR to whatever you think is best!

[luke-rucker]

Bump! It would be great if we could get this merged or if you could provide more feedback. Would really like to use this feature at work.

[jbielick]

I think I prefer the event emitter approach. I agree that does preclude the ability to block the shutdown and be subject to the hard shutdown timeout. If this is critical, maybe we can await all of the stop listeners? e.g. await Promise.all(worker.listeners('exit').map(f => f())))

I think the appropriate event name for after the workers are stopped is exit