@Override
    public String visit(SystemStreamPartition systemStreamPartition, StartpointSpecific startpointSpecific) {
      return startpointSpecific.getSpecificOffset();
    }
Offers a kafka specific implementation of {@link StartpointVisitor} that resolves
different types of {@link Startpoint} to samza offset.
samza-kafka/src/main/java/org/apache/samza/system/kafka/KafkaSystemAdmin.java