ONOS-2197 Adding some logic to keep track of expired IP Lease Assignments

Change-Id: I330e5fbf790f0a4fe72f1fbce272617837225e64
diff --git a/onos-app-dhcpserver/src/main/java/org/onosproject/dhcpserver/IPAssignment.java b/onos-app-dhcpserver/src/main/java/org/onosproject/dhcpserver/IPAssignment.java
index 56cf3da..1129d2a 100644
--- a/onos-app-dhcpserver/src/main/java/org/onosproject/dhcpserver/IPAssignment.java
+++ b/onos-app-dhcpserver/src/main/java/org/onosproject/dhcpserver/IPAssignment.java
@@ -126,6 +126,15 @@
     }
 
     /**
+     * Creates and returns a new builder instance that clones an existing IPAssignment.
+     *
+     * @return new builder
+     */
+    public static Builder builder(IPAssignment assignment) {
+        return new Builder(assignment);
+    }
+
+    /**
      * IPAssignment Builder.
      */
     public static final class Builder {