Nanny.close should gracefully close worker

[mrocklin]

Currently Nanny.close just terminates the worker process. Probably it should send a close signal

future = self.rpc(self.worker_address).terminate(nanny=False)

And then wait on that future for a suitable time

with ignoring(gen.TimeoutError):
    yield gen.with_timeout(timedelta(seconds=timeout/2), future)

I'm not sure that this gets the job done though. When I tried this there were still some lingering comms around.