Fix a work-counting bug introduced by the workqueue merge

David Goulet finds that when he runs a busy relay for a while with the
latest version of the git code, the number of onionskins handled
slowly dwindles to zero, with total_pending_tasks wedged at its
maximum value.

I conjecture this is because the total_pending_tasks variable isn't
decremented when we successfully cancel a job.  Fixed that.

Fixes bug 14741; bugfix not on any released version of tor.