SegmentWithState: Add toString method. (#5635)

The class appears in log messages, and the default toString method
isn't very informative.

server/src/main/java/io/druid/segment/realtime/appenderator/SegmentWithState.java

@@ -150,4 +150,13 @@ public class SegmentWithState
         to
     );
   }
+
+  @Override
+  public String toString()
+  {
+    return "SegmentWithState{" +
+           "segmentIdentifier=" + segmentIdentifier +
+           ", state=" + state +
+           '}';
+  }
 }