Kafka Pack should provide a default way of handling instance changes

i) The code to handle changes of Flowlet instances have to be implemented by the class extending KafkaConsumerFlowlet but a default way of handling it (sample logic is part of Kafka08ConsumingApp.KafkaSource).

Minor: If KafkaConsumerFlowet by default takes expects properties such as kafka.zookeeeper, kafka.brokers, kafka.topic, kafka.partitions with these keys, then I think it might reduce some code for the classes extending KafkaConsumerFlowlet.