proxy arp functional; needs testing

Change-Id: Ib948ec3eac07d2650becf720a9c3c5ca014a0994
diff --git a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
index 493580d..779e1ee 100644
--- a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
@@ -53,7 +53,7 @@
     protected final AbstractListenerRegistry<LinkEvent, LinkListener>
             listenerRegistry = new AbstractListenerRegistry<>();
 
-    private LinkStoreDelegate delegate = new InternalStoreDelegate();
+    private final LinkStoreDelegate delegate = new InternalStoreDelegate();
 
     private final DeviceListener deviceListener = new InternalDeviceListener();