Add support for length fields encoded as varints.
Add support for length fields encoded as varints.

Kafka 0.11 encodes Record length as varint.

vrischmann/sarama,Shopify/sarama,Shopify/sarama,VividCortex/sarama,mailgun/sarama,mailgun/sarama,vrischmann/sarama,DataDog/sarama,VividCortex/sarama,DataDog/sarama,remerge/sarama,remerge/sarama