Add protocol TxnOffsetCommit v0

[tulios]

The producer will send a TxnOffsetCommit to the consumer coordinator to persist the offsets in the __consumer-offsets topic. The consumer coordinator validates that the producer is allowed to make this request (and is not a zombie) by using the ID and producer epoch which are sent as part of this request.

The consumed offsets are not visible externally until the transaction is committed.

TxnOffsetCommit Request (Version: 0) => transactional_id group_id producer_id producer_epoch [topics] 
  transactional_id => STRING
  group_id => STRING
  producer_id => INT64
  producer_epoch => INT16
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset metadata 
      partition => INT32
      offset => INT64
      metadata => NULLABLE_STRING

TxnOffsetCommit Response (Version: 0) => throttle_time_ms [topics] 
  throttle_time_ms => INT32
  topics => topic [partitions] 
    topic => STRING
    partitions => partition error_code 
      partition => INT32
      error_code => INT16

https://kafka.apache.org/protocol.html#The_Messages_TxnOffsetCommit

[ianwsperber]

Starting work

[tulios]

Edit:
closed the issue