Make sure that HostLocation's timestamp is updated

Make sure that the most recent timestamp of location is recorded in DhcpStore when this location is already present in locations set.

Change-Id: Idfc768d9812ef3cb01cc8a7a3d7778a3e3bd5caf
(cherry picked from commit bd1472dc540d5aa61d4dd9bf7b4705cf5ff7f3d8)
diff --git a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DhcpRecord.java b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DhcpRecord.java
index e4c2673..f0beff9 100644
--- a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DhcpRecord.java
+++ b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DhcpRecord.java
@@ -95,6 +95,9 @@
      * @return the DHCP record
      */
     public DhcpRecord addLocation(HostLocation location) {
+        if (locations.contains(location)) {
+            locations.remove(location);
+        }
         locations.add(location);
         return this;
     }