default boolean isSystemMessage(KV<Object, Object> message) {
    return false;
  }
Determine if the input samza message is a system message.
This API will soon be removed when descriptor creation is done by SamzaRelConverter.
@param message input samza message.
@return true if the input message is system message.
samza-sql/src/main/java/org/apache/samza/sql/interfaces/SamzaRelConverter.java