Support FetchRequest v4 / Message Format v2

[dpkp]

In Kafka 0.11, the structure of the 'MessageSet' and 'Message' were significantly
changed. Not only were new fields added to support new features like exactly once
semantics and record headers, but the recursive nature of the previous versions of
the message format was eliminated in favor of a flat structure. A 'MessageSet' is
now called a 'RecordBatch', which contains one or more 'Records' (and not 'Messages').
When compression is enabled, the RecordBatch header remains uncompressed,
but the Records are compressed together. Further, multiple fields in the 'Record'
are varint encoded, which leads to significant space savings for larger batches.

See https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol#AGuideToTheKafkaProtocol-Messagesets

[tvoinarovskyi]

fml, started poking with the new format. If we start using it the introduced CRC32C will be a problem probably.
See https://issues.apache.org/jira/browse/KAFKA-1449 for more info.
tl;dr The v2 Message format uses a different CRC32 implementation.

[tvoinarovskyi]

A bit of status report here. I finished a pure python prototype on top of aiokafka, currently checking compatibility and tests. On next week plan to port it to kafka-python. Thou I think this will require a C extension to be relatively fast (pure python Crc32C and varint coder can be made much faster), did not measure yet thou.

[tvoinarovskyi]

I think this can be closed already