Extends XConnectManager

- Listens for L2LbEvent
- Leverages a cache to wait l2lb nextid

Includes a small change in the L2LbService interface

Change-Id: Id545e2228dde013620b4b415d1d2619027ef15d2
diff --git a/apps/l2lb/src/main/java/org/onosproject/l2lb/api/L2LbService.java b/apps/l2lb/src/main/java/org/onosproject/l2lb/api/L2LbService.java
index db5f20c..ce396ac 100644
--- a/apps/l2lb/src/main/java/org/onosproject/l2lb/api/L2LbService.java
+++ b/apps/l2lb/src/main/java/org/onosproject/l2lb/api/L2LbService.java
@@ -56,7 +56,7 @@
      * @param key L2 load balancer key
      * @return next ID
      */
-    int getL2LbNexts(DeviceId deviceId, int key);
+    int getL2LbNext(DeviceId deviceId, int key);
 
     /**
      * Reserves a l2 load balancer. Only one application