Consumer: handle compressed relative offsets

[eapache]

New message format does something weird with these. See
https://cwiki.apache.org/confluence/display/KAFKA/KIP-31+-+Move+to+relative+offsets+in+compressed+message+sets

Fixes #720. Supercedes #721. Thanks to @dynamix for the first draft of the fix.

@dynamix @Dieterbe could you verify this fix works correctly?

cc @wvanbergen

[Dieterbe]

hey, when i rebuild my tool with your branch and send it dummy data, snappy compressed, the first time it actually seemed to work, but in subsequent tries i seemed to get the following again:

[Sarama] 2016/08/25 08:40:00 kafka: error while consuming mdm/0: kafka: response did not contain all the expected topic/partition blocks
[Sarama] 2016/08/25 08:40:00 consumer/broker/0 abandoned subscription to mdm/0 because kafka: response did not contain all the expected topic/partition blocks
[Sarama] 2016/08/25 08:40:02 consumer/mdm/0 finding new broker
[Sarama] 2016/08/25 08:40:02 client/metadata fetching metadata for [mdm] from broker kafka:9092
[Sarama] 2016/08/25 08:40:02 consumer/broker/0 added subscription to mdm/0
[Sarama] 2016/08/25 08:40:02 kafka: error while consuming mdm/0: kafka: response did not contain all the expected topic/partition blocks
[Sarama] 2016/08/25 08:40:02 consumer/broker/0 abandoned subscription to mdm/0 because kafka: response did not contain all the expected topic/partition blocks

However i was also having some other issues with our dev-stack. Will need to do more proper testing.

[dynamix]

Currently testing this in combination with the consumer group stuff I'm working on. This looks better than my initial fix - which was indeed incorrect and did lead to invalid offsets. So far it seems to work. 👍