Add facade object called UnifiedDiscreteResources

This is for ONOS-4281

Change-Id: I04649932fdb8983492878732a04c93a21b4cafe7
(cherry picked from commit ac6ff6e1984e0351938467be7677072888258c7d)
diff --git a/core/store/dist/src/main/java/org/onosproject/store/resource/impl/UnifiedDiscreteResources.java b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/UnifiedDiscreteResources.java
new file mode 100644
index 0000000..ec121cd
--- /dev/null
+++ b/core/store/dist/src/main/java/org/onosproject/store/resource/impl/UnifiedDiscreteResources.java
@@ -0,0 +1,82 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.store.resource.impl;
+
+import org.onosproject.net.resource.DiscreteResource;
+import org.onosproject.net.resource.DiscreteResourceId;
+
+import java.util.List;
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * Represents a set of resources containing resources that can be encoded as integer
+ * and those that can't encoded as integer.
+ */
+final class UnifiedDiscreteResources implements DiscreteResources {
+    private final DiscreteResources nonEncodables;
+
+    static DiscreteResources empty() {
+        return new UnifiedDiscreteResources();
+    }
+
+    static DiscreteResources of(List<DiscreteResource> resources) {
+        return new UnifiedDiscreteResources(resources);
+    }
+
+    private UnifiedDiscreteResources() {
+        this.nonEncodables = NonEncodableDiscreteResources.empty();
+    }
+
+    private UnifiedDiscreteResources(List<DiscreteResource> resources) {
+        this.nonEncodables = NonEncodableDiscreteResources.of(resources);
+    }
+
+    @Override
+    public Optional<DiscreteResource> lookup(DiscreteResourceId id) {
+        return nonEncodables.lookup(id);
+    }
+
+    @Override
+    public DiscreteResources difference(DiscreteResources other) {
+        return nonEncodables.difference(other);
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return nonEncodables.isEmpty();
+    }
+
+    @Override
+    public boolean containsAny(List<DiscreteResource> other) {
+        return nonEncodables.containsAny(other);
+    }
+
+    @Override
+    public DiscreteResources add(DiscreteResources other) {
+        return nonEncodables.add(other);
+    }
+
+    @Override
+    public DiscreteResources remove(List<DiscreteResource> removed) {
+        return nonEncodables.remove(removed);
+    }
+
+    @Override
+    public Set<DiscreteResource> values() {
+        return nonEncodables.values();
+    }
+}