Read the latest value of dispatchFuture

The field, dispatchFuture, is accessed in activate() method and the thread
where Watchdog is running. Then, different threads could access the field
and couldn't read the latest value.

Change-Id: Iceb390bcf3c5711c1dbb59198667138c8a90b036

@@ -75,7 +75,7 @@ public class CoreEventDispatcher extends DefaultEventSinkRegistry
     private TimerTask watchdog;
     private EventSink lastSink;
     private final Stopwatch stopwatch = Stopwatch.createUnstarted();
-    private Future<?> dispatchFuture;
+    private volatile Future<?> dispatchFuture;
 
     @Override
     public void post(Event event) {