Simplified Downloader (positive) termination condition

Simplified Downloader (positive) termination condition

With JobWait API used by the callers, we no longer need to rely on some
unrelated async job call noticing the callback cancellation. We are now
guaranteed to receive a noteAbort() call.

I also removed a call to always-true parent doneAll() because no other
job, not even a parent's job can force Downloader to keep running -- if
we have notified the requestor, we are done! Squid code is not
consistent in application of that principle (yet).