fix some of findbugs issues

Change-Id: I20aa54af16f3a1e3323d735fe53cc26c03d5e52e
diff --git a/utils/netty/src/main/java/org/onlab/netty/NettyMessagingService.java b/utils/netty/src/main/java/org/onlab/netty/NettyMessagingService.java
index 5ef1768..26d835d 100644
--- a/utils/netty/src/main/java/org/onlab/netty/NettyMessagingService.java
+++ b/utils/netty/src/main/java/org/onlab/netty/NettyMessagingService.java
@@ -163,6 +163,7 @@
         handlers.putIfAbsent(type, handler);
     }
 
+    @Override
     public void unregisterHandler(String type) {
         handlers.remove(type);
     }
@@ -242,7 +243,7 @@
         }
     }
 
-    private class WriteTask implements Runnable {
+    private static class WriteTask implements Runnable {
 
         private final InternalMessage message;
         private final Channel channel;