ONOS-1871: Send Topology MastershipEvent info to Topology listeners.

The MastershipEvent info is needed by applications such as the GUI
(via the Websocket client).

NOTE: Previously, the Mastership Events were sent only when there
was any change, but no Mastership Events were sent when a listener
has just subscribed.

The solution is to add a mechanism for creating and sending Topology
Snapshot event to new listeners, and that event also includes
the Mastership Events.

The modifications are:

 * Renamed and updated the ITopologyService API for adding/removing
   Topology listeners:
   OLD: registerTopologyListener() and deregisterTopologyListener()
   NEW: addListener() and removeListener()

   Also, addListener() has a second argument:
       "boolean startFromSnapshot"
   If that argument is true, and if the topology is not empty, the first
   (expected) event to that listener should be a snapshot of the current
   topology.

 * Added TopologyEvents() constructor for ADDED events only. Such event
   can be used to represent a snapshot of the topology.

 * Added new APIs to TopologyInternal:
   getAllSwitchEvents(), getAllPortEvents(), get AllLinkEvents(),
   getAllHostEvents()
   Those APIs are needed for creating a snapshot of the topology.

 * Added a mechanism for creating empty (NO-OP) TopologyEvent instance,
   and use that mechanism to "wake-up" the EventHandler processing thread
   when it needs to send Topology Snapshot to a new listener.
   This solution is (kind-of) a hack.

Change-Id: Ie1eb52242f58682aac61f54af29c3b5d291ac0bd
diff --git a/src/main/java/net/onrc/onos/core/topology/TopologyModule.java b/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
index d24f4df..fee98b4 100644
--- a/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
+++ b/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
@@ -77,12 +77,13 @@
     }
 
     @Override
-    public void registerTopologyListener(ITopologyListener listener) {
-        topologyManager.registerTopologyListener(listener);
+    public void addListener(ITopologyListener listener,
+                            boolean startFromSnapshot) {
+        topologyManager.addListener(listener, startFromSnapshot);
     }
 
     @Override
-    public void deregisterTopologyListener(ITopologyListener listener) {
-        topologyManager.deregisterTopologyListener(listener);
+    public void removeListener(ITopologyListener listener) {
+        topologyManager.removeListener(listener);
     }
 }