add toString

Change-Id: I2a38b27164c4c4c0e259a32dbb283dd384b61c26
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/service/WriteResult.java b/core/store/dist/src/main/java/org/onlab/onos/store/service/WriteResult.java
index fa20a04..aec3046 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/service/WriteResult.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/service/WriteResult.java
@@ -1,5 +1,7 @@
 package org.onlab.onos.store.service;
 
+import com.google.common.base.MoreObjects;
+
 
 /**
  * Database write result.
@@ -27,4 +29,13 @@
     public VersionedValue previousValue() {
         return previousValue;
     }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                .add("tableName", tableName)
+                .add("key", key)
+                .add("previousValue", previousValue)
+                .toString();
+    }
 }