This is a refactor aimed at cleaning up and simplifying the existing
job queue.
As of now, all jobs are cancelled at the same time and in the same
way, so this commit removes the per-job cancellation token. If the
need for such support is demonstrated, support can be re-added.
* Revise documentation for ClosureCounter and Workers.
* Simplify code, removing unnecessary function arguments and
deduplicating expressions
* Restructure job handlers to no longer need to pass a job's
handle to the job.
If the JobQueue is used during shutdown then those Jobs may access
Stoppables after they have already stopped. This violates the
preconditions of Stoppables and may lead to undefined behavior.
The solution taken here is to reference count all Jobs in the
JobQueue. At stop time all Jobs already in the JobQueue are
allowed to run to completion, but no further Jobs are allowed
into the JobQueue.
If a Job is rejected from the JobQueue (because we are stopping),
then JobQueue::addJob() returns false, so the caller can make any
necessary adjustments.
