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1. After most of the consumers are upgraded, Bump up message.format.version=1 and rolling bounce the brokers.
   
2. Upgraded brokers do the followings:
   
   1. When broker sees a producer request V1 (MessageAndOffset = V0), it will decompress the message, assign offsets using definitive relative offsets and NOT re-compress the message. i.e. upconvert the message format to MessageAndOffset V1.
      
   2. When broker sees a producer request V2 (MessageAndOffset = V1), it will decompress the message, assign offsets using definitive relative offsets and NOT do re-compression.  i.e. downconvert the message format to MessageAndOffset V0.
   3. When broker sees a fetch request V1 (Supporting MessageAndOffset = V0), because the data format on disk is MessageAndOffset V0V1, it will NOT use the zero-copy transfer to reply with fetch response V1 with MessageAndOffset V0.
      
   4. When broker sees a fetch request V2 (Supporting MessageAndOffset = V0, V1), because the data format on disk is MessageAndOffset V0V1, it will use zero-copy transfer to reply with fetch response V2 with MessageAndOffset V0V1.

For producer, there will be no impact.

In phase 1, there will be no impact for consumers.

In phase 2, there will be some performance penalty for consumers that only supports MessageAndOffset V0, there will be some performance penalty because there is no zero-copy transfer.

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