Parsing only known control batches value
Parsing only known control batches value

Kafka documentation defines control batches as a single record batches
containing data that shouldn't be passed to applications. The record
contained inside the control batch consist of a key with well defined
format and a value which format is defined by the control batch type.

Control batch key format:
```
ControlRecordKey => version type
  version => INT16
  type => INT16
```

Fixed control batch parsing logic by first parsing the key to check if
control batch is of known type and only then parsing corresponding
record value.

Signed-off-by: Michal Maslanka <a2df1e659c9fd2578de0a26565357cb273292eeb@vectorized.io>

Shopify/sarama,DataDog/sarama,DataDog/sarama,Shopify/sarama