@Override
  public KV<Object, Object> convertToSamzaMessage(SamzaSqlRelMessage relMessage) {
    return convertToSamzaMessage(relMessage, this.payloadSchema);
  }
Convert the nested relational message to the output samza message.
samza-sql/src/main/java/org/apache/samza/sql/avro/AvroRelConverter.java