make scheduler shutdown more stringent

- run closeScheduler upon ActorSystem termination (directly)
- this will execute all outstanding tasks (dispatcher shutdowns have
  been queued already, because the last actor has already exited)
- further use of the scheduler (e.g. from tasks just being run) results
  in IllegalStateException
- catch such exceptions in DefaultPromise&MessageDispatcher in case of
  self-rescheduling tasks and execute final action immediately
- also silently stop recurring tasks

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -212,7 +212,9 @@ abstract class MessageDispatcher(val prerequisites: DispatcherPrerequisites) ext
           }
         case RESCHEDULED ⇒
           if (shutdownScheduleUpdater.compareAndSet(MessageDispatcher.this, RESCHEDULED, SCHEDULED))
-            scheduler.scheduleOnce(this, shutdownTimeout)
+            try scheduler.scheduleOnce(this, shutdownTimeout) catch {
+              case _: IllegalStateException ⇒ shutdown()
+            }
           else run()
       }
     }