[SDFAB-713] Ensure TrafficMonitor is properly shutdown

This fixes also a potential leak of memory and thread

Change-Id: I266a37365cef3776463a1aca2c1e2717507b4f30
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/topo/Topo2TrafficMessageHandler.java b/web/gui/src/main/java/org/onosproject/ui/impl/topo/Topo2TrafficMessageHandler.java
index a6e0e9d..9c84563 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/topo/Topo2TrafficMessageHandler.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/topo/Topo2TrafficMessageHandler.java
@@ -98,6 +98,12 @@
     }
 
     @Override
+    public void destroy() {
+        super.destroy();
+        traffic2.stopMonitoring();
+    }
+
+    @Override
     protected Collection<RequestHandler> createRequestHandlers() {
         return ImmutableSet.of(
                 new Topo2AllTraffic(),