1.The situation is:（execute this in cluster env） IoTDB> merge
Msg: 500: serialization unimplemented 2.The essential reason is : MergePlan doesn't implement its own serialize method, therefore, this problem happened. 3. Discuss: should it implement its own serialize method or not at this moment? More related description or information at https://issues.apache.org/jira/browse/IOTDB-1075
