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
A {@link FutureCallback} for ZK operations so that if an operation failed due
to {@link KeeperException.Code#NONODE} error, it will watch for the existence of the given node and perform
a given action when the node become available. Upon node removal, the node will again be watched for creation
and will trigger the given action again once it is created.