Get Kafka offsets from consumer group in ZK when using direct stream

This is the proposal.

The simpler direct API (the one that does not take explicit offsets) can be modified to also pick up the initial offset from ZK if group.id is specified. This is exactly similar to how we find the latest or earliest offset in that API, just that instead of latest/earliest offset of the topic we want to find the offset from the consumer group. The group offsets is ZK is not used at all for any further processing and restarting, so the exactly-once semantics is not broken.

The use case where this is useful is simplified code upgrade. If the user wants to upgrade the code, he/she can the context stop gracefully which will ensure the ZK consumer group offset will be updated with the last offsets processed. Then the new code is started (not restarted from checkpoint) can pickup the consumer group offset from ZK and continue where the previous code had left off.

Without the functionality of picking up consumer group offsets to start (that is, currently) the only way to do this is for the users to save the offsets somewhere (file, database, etc.) and manage the offsets themselves. I just want to simplify this process.