minor Kryo related cleanup

Change-Id: I772fc1f776563d30f1a4081c6fddd5739d985263
diff --git a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/MastershipRoleSerializer.java b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/MastershipRoleSerializer.java
index 3903491..dab5aa8 100644
--- a/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/MastershipRoleSerializer.java
+++ b/core/store/serializers/src/main/java/org/onlab/onos/store/serializers/MastershipRoleSerializer.java
@@ -12,6 +12,14 @@
  */
 public class MastershipRoleSerializer extends Serializer<MastershipRole> {
 
+    /**
+     * Creates {@link MastershipRole} serializer instance.
+     */
+    public MastershipRoleSerializer() {
+        // non-null, immutable
+        super(false, true);
+    }
+
     @Override
     public MastershipRole read(Kryo kryo, Input input, Class<MastershipRole> type) {
         final String role = kryo.readObject(input, String.class);