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. 
Anyway it is an interesting API design question, let's discuss that tomorrow :-)