fix(ThreadedQueue): ensure processed matches task_done
fix(ThreadedQueue): ensure processed matches task_done

There's a bug where it's possible for the main thread to proceed after
queue.join() stops blocking butbefore self.processed is incremented,
resulting in a temporary discrepancy of 1. We invert the order of these
operations to ensure that join will not stop blocking until all tasks
are accounted for.

seung-lab/cloud-volume,seung-lab/cloud-volume,seung-lab/cloud-volume