Avoid extracting resoure set from backing range set

This is one of fixes for ONOS-4684

Change-Id: I5192df366d16b8aad5f8987abbf539897ca0e51c
diff --git a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EncodedDiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EncodedDiscreteResources.java
index 5ccb6d2..d02e91c 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EncodedDiscreteResources.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/EncodedDiscreteResources.java
@@ -93,6 +93,15 @@
         return new EncodedDiscreteResources(newRangeSet, this.codec);
     }
 
+    EncodedDiscreteResources add(EncodedDiscreteResources other) {
+        checkArgument(this.codec.getClass() == other.codec.getClass());
+
+        RangeSet<Integer> newRangeSet = TreeRangeSet.create(this.rangeSet);
+        newRangeSet.addAll(other.rangeSet);
+
+        return new EncodedDiscreteResources(newRangeSet, this.codec);
+    }
+
     boolean isEmpty() {
         return rangeSet.isEmpty();
     }