Add support for tagged fields

[swenzel]

Kafka has implemented a feature for so called "tagged fields".
Those are supposed to reduce the amount of data that needs to be sent between client and server and to allow for more dynamic schema evolution.
With these fields it is possible to request or provide only specific fields/data and thereby eliminating the need to send unimportant information just to comply with the protocol.
Unfortunately tagged fields are currently not well documented at http://kafka.apache.org/protocol
Some more information can be found here: https://cwiki.apache.org/confluence/display/KAFKA/KIP-482%3A+The+Kafka+Protocol+should+Support+Optional+Tagged+Fields