adding group garbage collection functionality

If a group has a reference count of zero for more than
a configurable timeout, it is garbage collected.

This feature can be deactivated by component config.

Change-Id: I254d62a90ef7ac8d2ce2f406b67957455a5bf4d0
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 1321893..f8048d4 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
@@ -156,6 +156,11 @@
         }
 
         @Override
+        public int age() {
+            return 0;
+        }
+
+        @Override
         public Type type() {
             return GroupDescription.Type.ALL;
         }