Clean up triggers awaiting global state modification at shutdown

Otherwise they can be reported as memory leaks since no more global
state modifications will take place to notify the trigger to clean
itself up.

src/Trigger.h

@@ -62,6 +62,10 @@ public:
 	// later to avoid race conditions.
 	void Modified(notifier::Modifiable* m) override
 		{ QueueTrigger(this); }
+	// Overridden from notifer::Receiver.  If we're still waiting
+	// on an ID/Val to be modified at termination time, we can't hope
+	// for any further progress to be made, so just Unref ourselves.
+	void Terminate() override;
 
 	const char* Name() const;