Added new notification framework:
 * The listener has to implement the IEventChannelListener interface.
 * A listener subscribes to a notification channel by using
   IDatagridService.addListener(), and unsubscribes by using
   IDatagridService.removeListener()
   The channel is created and started automatically when the first
   listener is added.
 * A channel can be created by using IDatagridService.createChannel()
   e.g., by the publisher of events. Note that createChannel() automatically
   starts the event channel operation.

 * A publisher uses IEventChannel.addEntry() and removeEntry() to generate
   add/delete events.

 * The listener receives the add/remove events by implementing
   the IEventChannelListener.entryAdded() and entryRemoved() methods.

Example of usage:

Listener/Subscriber:

    private FooFlowPath fooFlowPath = new FooFlowPath();

    datagridService.addListener("mapFooFlowPath", fooFlowPath,
                                   Long.class, FlowPath.class);
...

    class FooFlowPath implements IEventChannelListener<Long, FlowPath> {
        /**
         * Receive a notification that an entry is added.
         *
         * @param value the value for the entry.
         */
        @Override
        public void entryAdded(FlowPath value) {
            // Process the event
        }

        /**
         * Receive a notification that an entry is removed.
         *
         * @param value the value for the entry.
         */
        @Override
        public void entryRemoved(FlowPath value) {
            // Process the event
        }
        ...
    }

Sender/Publisher:

    private IEventChannel<Long, FlowPath> fooFlowPathChannel = null;
    fooFlowPathChannel = datagridService.createChannel("mapFooFlowPath",
            Long.class, FlowPath.class);
    ...

    // Transmit an event
    fooFlowPathChannel.addEntry(flowPath.flowId().value(), flowPath);

Change-Id: Ie3246a4e200d5b6293c1f175df3652cdf571be69
diff --git a/src/main/java/net/onrc/onos/datagrid/IDatagridService.java b/src/main/java/net/onrc/onos/datagrid/IDatagridService.java
index d3cf98e..118cbfa 100755
--- a/src/main/java/net/onrc/onos/datagrid/IDatagridService.java
+++ b/src/main/java/net/onrc/onos/datagrid/IDatagridService.java
@@ -21,6 +21,45 @@
  * Interface for providing Datagrid Service to other modules.
  */
 public interface IDatagridService extends IFloodlightService {
+    /**
+     * Create an event channel.
+     *
+     * If the channel already exists, just return it.
+     * NOTE: The channel is started automatically.
+     *
+     * @param channelName the event channel name.
+     * @param typeK the type of the Key in the Key-Value store.
+     * @param typeV the type of the Value in the Key-Value store.
+     * @return the event channel for the channel name.
+     */
+    <K, V> IEventChannel<K, V> createChannel(String channelName,
+					     Class<K> typeK, Class<V> typeV);
+
+    /**
+     * Add event channel listener.
+     *
+     * NOTE: The channel is started automatically right after the listener
+     * is added.
+     *
+     * @param channelName the event channel name.
+     * @param listener the listener to add.
+     * @param typeK the type of the Key in the Key-Value store.
+     * @param typeV the type of the Value in the Key-Value store.
+     * @return the event channel for the channel name.
+     */
+    <K, V> IEventChannel<K, V> addListener(String channelName,
+			   IEventChannelListener<K, V> listener,
+			   Class<K> typeK, Class<V> typeV);
+
+    /**
+     * Remove event channel listener.
+     *
+     * @param channelName the event channel name.
+     * @param listener the listener to remove.
+     */
+    <K, V> void removeListener(String channelName,
+			      IEventChannelListener<K, V> listener);
+
     /*
      * register all the intents as one batch
      */