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
diff --git a/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java b/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
index 1647eff..26a89d4 100644
--- a/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
+++ b/core/net/src/main/java/org/onosproject/event/impl/CoreEventDispatcher.java
@@ -75,7 +75,7 @@
     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) {