Support RecordBatch

[emmettbutler]

Since Kafka 0.11, the MessageSet format has had an updated version called RecordBatch. We should investigate what it would take to support this format, and what the benefits would be. This will probably require significant changes within protocol.py to intelligently switch between MessageSet implementations based on Kafka broker version.

[emmettbutler]

There is also a change to MessageSet in 0.10 (the addition of timestamp) that should be supported.