Signal that the executor will no longer accept job submission.
		
		Worker threads/processes are now allowed to terminate after all
		jobs have been are completed.  Without a call to close(), they will
		stay around forever waiting for more jobs to come.

def close(self):
		"""Signal that the executor will no longer accept job submission.
		
		Worker threads/processes are now allowed to terminate after all
		jobs have been are completed.  Without a call to close(), they will
		stay around forever waiting for more jobs to come.
		"""
		with self.lock:
			if self.closed:
				return
			self.waitqueue.put(StopIteration)
			self.closed = True