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/core/api/src/main/java/org/onosproject/net/group/Group.java b/core/api/src/main/java/org/onosproject/net/group/Group.java
index 5440775..d053029 100644
--- a/core/api/src/main/java/org/onosproject/net/group/Group.java
+++ b/core/api/src/main/java/org/onosproject/net/group/Group.java
@@ -96,4 +96,12 @@
      * @return number of flow rules or other groups pointing to this group
      */
     long referenceCount();
+
+    /**
+     * Obtains the age of a group. The age reflects the number of polling rounds
+     * the group has had a reference count of zero.
+     *
+     * @return the age of the group as an integer
+     */
+    int age();
 }