@VisibleForTesting
  boolean isStarted() {
    return isStarted;
  }
Helper method that sends a series of SetConfig messages to the coordinator
stream.

@param source
         An identifier to denote which source is sending a message. This
         can be any arbitrary string.
@param config
         The config object to store in the coordinator stream.
samza-core/src/main/java/org/apache/samza/coordinator/stream/CoordinatorStreamSystemProducer.java