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.
