First complete version of Kafka flowlet

Quick links:

README: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/README.md

BUILD: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/BUILD.md
Persists the offset for each {@link TopicPartition} to a {@link KeyValueTable} provided by
{@link #getOffsetStore()}. The key is simply a concatenation of
topic, partition, and brokerId. The value is a 8-bytes encoded long of the offset. If no dataset is provided,
this method is a no-op.