Clear task queue when Scheduler is shutdown

Tasks on scheduler thread. In ``DynamoScheduler.Shutdown()``, it notifies schedule thread to shutdown as well. Depending on scheduler thread's implementation, it is possible that although scheduler thread have been unavailable, but there are still some pending tasks in the queue. For example, ``RevitSchedulerThread`` uses Revit's idle event to process tasks, and if it is detached from idle event, tasks would never get a chance to run, so we need to make sure all tasks are cleared. 

@Benglin  please. 
No I didn't. :-) I just read the implementation of ``Dispatcher``, It doesn't prohibit you from calling ``Invoke()`` or ``BeginInvoke()`` after it is shutdown (at least from the code, but i'm not sure what happen at runtime). So probably it does nothing if you try to send more tasks to it.