@Override
  public void flush(String source) {
    // nothing to do
  }
If the SystemProducer buffers messages before sending them to its underlying system, it should flush those
messages and leave no messages remaining to be sent.

@param source String representing the source of the message.
samza-core/src/main/java/org/apache/samza/system/inmemory/InMemorySystemProducer.java