Compaction of discrete resources with range based representation

This resolves ONOS-4281

Change-Id: I0739ba94cc0b3ce617e2db44307fef396dcfb942
(cherry picked from commit 34e2c1c223ec62c85c2613c0d899a0cc06b63f33)
diff --git a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
index 6b7da8e..3b39120 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/DiscreteResources.java
@@ -32,7 +32,7 @@
      * @return a empty set.
      */
     static DiscreteResources empty() {
-        return UnifiedDiscreteResources.empty();
+        return EmptyDiscreteResources.INSTANCE;
     }
 
     /**
@@ -41,7 +41,7 @@
      * @param resources resources
      * @return instance
      */
-    static DiscreteResources of(List<DiscreteResource> resources) {
+    static DiscreteResources of(Set<DiscreteResource> resources) {
         return UnifiedDiscreteResources.of(resources);
     }