add hashCode, etc. to MessageSubject

Change-Id: I5c0185ea6bc8ab37fc5bade1c64cc30be6a79f9a

 package org.onlab.onos.store.cluster.messaging;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import java.util.Objects;
+
 /**
  * Representation of a message subject.
  * Cluster messages have associated subjects that dictate how they get handled
@@ -10,7 +14,7 @@ public class MessageSubject {
     private final String value;
 
     public MessageSubject(String value) {
-        this.value = value;
+        this.value = checkNotNull(value);
     }
 
     public String value() {
@@ -21,4 +25,24 @@ public class MessageSubject {
     public String toString() {
         return value;
     }
+
+    @Override
+    public int hashCode() {
+        return value.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+        if (obj == null) {
+            return false;
+        }
+        if (getClass() != obj.getClass()) {
+            return false;
+        }
+        MessageSubject that = (MessageSubject) obj;
+        return Objects.equals(this.value, that.value);
+    }
 }