Shut down the Executor.  Suspend all waiting jobs.
		
		Running workers will terminate after finishing their current job items.
		The call will block until all workers are terminated.

def shutdown(self):
		"""Shut down the Executor.  Suspend all waiting jobs.
		
		Running workers will terminate after finishing their current job items.
		The call will block until all workers are terminated.
		"""
		with self.lock:
			self.pool.inqueue.put(StopIteration)   # Stop the pool workers
			self.waitqueue.put(StopIteration)      # Stop the input_feeder
			_iterqueue(self.waitqueue) >> item[-1] # Exhaust the waitqueue
			self.closed = True
		self.join()