* Wrap calls to PollScheduler._schedule_tasks() with a _schedule() method

* Wrap calls to PollScheduler._schedule_tasks() with a _schedule() method
  that automatically returns early from any recursive calls that the
  _schedule_tasks() call might trigger. This makes _schedule() safe to call from
  inside exit listeners.

* Call _schedule() inside Scheduler._job_exit() to trigger a new job when
  possible.

* Fix SequentialTaskQueue._task_exit() to avoid a potential KeyError if the
  task had already been removed from self.running_tasks for some reason.

svn path=/main/trunk/; revision=11030