Clear out plab evproxy subscriptions when the event scheduler is stopped.

	* event/sched/event-sched.c: Add an __ns_teardown sequence that
	can be used to send events when the scheduler is stopped.

	* event/tbgen/tevc.c: Add a timeout flag that can be used to bound
	the time spent waiting for an event to complete.

	* tbsetup/eventsys.proxy.in: When stopping/replaying, run the
	__ns_teardown sequence and wait for it to complete.