fetcher: Skip transactional control batches

[JayH5]

Back-port support for transactional control messages in kafka-python. Although longer-term we want to switch all consumers to confluent-kaka, this quick fix means that kafka-python-based consumers at least shouldn't crash if there are ever transactional producers writing to a consumed topic.

Note that this does not mean that the consumer supports transactions--there is still no support for the read_committed isolation.level (more here).

This code comes from dpkp#2361, original author bradenneal1. I didn't have a better way to write this myself. Unfortunately, writing a test for this would be quite difficult with the current test infrastructure. We have a history of back-porting stuff like this without tests, though (e.g. #71).

[JayH5]

We're a bit short of Yelp-org members today. I've received a +1 from abareghi [at] yelp [dot] com