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.
The two active users of DeadlineTimer, NetworkOPs and Application,
now use asio::steady_timers rather than DeadlineTimer.
DeadlineTimer is removed since it is no longer used.
To assure that all in-flight closures on timers are done before
Stoppables call stopped(), the JobCounter is made more generic.
It's now a ClosureCounter. The ClosureCounter is currently used
to count closures in flight for the JobQueue, NetworkOPs, and the
Application.