Fix: shut down the message dispatch thread gracefully when the channel is inactive.

At present, the message dispatch task is an infinte loop without return, the thread will nerver be destroyed even the channel is disconnected.

Change-Id: I0e8666e080677f1d549cec5e31f22b1e32f3a16a
diff --git a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OFChannelHandler.java b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OFChannelHandler.java
index cbcbf58..9be0b53 100644
--- a/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OFChannelHandler.java
+++ b/protocols/openflow/ctl/src/main/java/org/onosproject/openflow/controller/impl/OFChannelHandler.java
@@ -1266,7 +1266,7 @@
                  getSwitchInfoString());
 
         if (dispatcher != null) {
-            dispatcher.shutdown();
+            dispatcher.shutdownNow();
         }
 
          if (thisdpid != 0) {