Bugfix in routing logic to avoid null groups; Improvement in group handling.

Two main changes for the bug-fix:
    - avoid doing a full-reroute in the event of link failure as this will leave behind stale state in stores
      if event has been preceded by mastership change leading to the nuking of the ecmpSpg for one of the
      link's devices; instead do a rehash
    - when full-reroute is attempted, do it only once, with a complete nuke of the next-obj store

Improvement in group handling allows for a max number of retries for a group that failed to be added.
Earlier behavior was to try only once, and if it fails, it gets removed from the group-store. Now it
is removed after a couple of retries - so total 3 attempts to program the group.

Change-Id: I54ca8203cef779463522b01353540d12f8be3c91
diff --git a/web/api/src/test/java/org/onosproject/rest/resources/GroupsResourceTest.java b/web/api/src/test/java/org/onosproject/rest/resources/GroupsResourceTest.java
index 2cdb6ff..e13784d 100644
--- a/web/api/src/test/java/org/onosproject/rest/resources/GroupsResourceTest.java
+++ b/web/api/src/test/java/org/onosproject/rest/resources/GroupsResourceTest.java
@@ -189,6 +189,11 @@
         public GroupBuckets buckets() {
             return this.buckets;
         }
+
+        @Override
+        public int failedRetryCount() {
+            return 0;
+        }
     }
 
     /**