General link discovery module cleanup.

* Removed all events we don't use from the event system (switch and port
  events, link updates)
* Refactored event interface to two separate methods to match the other event
  interfaces
* Removed the LDUpdate class and moved all the event enums and classes to be
  internal to the LinkDiscoveryManager
* Removed all LinkTypes we no longer used and moved the one remaining type
  to the ILinkDiscoveryService. After this the ILinkDiscovery interface is
  no longer needed.
* Made Link immutable
* Removed the linkdiscovery.internal package as it only contained one class
* Readability improvements to LinkDiscoveryManager

Change-Id: Ifae97879aadc49b70a7b3d2294dcc540538c2cfc
diff --git a/src/main/java/net/onrc/onos/core/linkdiscovery/Link.java b/src/main/java/net/onrc/onos/core/linkdiscovery/Link.java
index db67cd5..2d1508f 100644
--- a/src/main/java/net/onrc/onos/core/linkdiscovery/Link.java
+++ b/src/main/java/net/onrc/onos/core/linkdiscovery/Link.java
@@ -24,11 +24,11 @@
 import org.codehaus.jackson.map.annotate.JsonSerialize;
 import org.openflow.util.HexString;
 
-public class Link {
-    private long src;
-    private short srcPort;
-    private long dst;
-    private short dstPort;
+public final class Link {
+    private final long src;
+    private final short srcPort;
+    private final long dst;
+    private final short dstPort;
 
 
     public Link(long srcId, short srcPort, long dstId, short dstPort) {