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MessageSet => 
  FirstOffset => int64
  Length => int32
  CRC => int32
  Magic => int8  /* bump up to “2” */
  Attributes => int16
  LastOffsetDelta => int32 {NEW}
  FirstTimestamp => int64 {NEW}
  MaxTimestamp => int64 {NEW}
  PID => int64 {NEW}
  Epoch => int16 {NEW}
  FirstSequence => int32 {NEW}
  Messages => Message1, Message2, … , MessageN {NEW}

Message => {ALL FIELDS NEW}
  Length => uintVar
  Attributes => int8
  TimestampDelta => intVar
  OffsetDelta => uintVar
  KeyLen => uintVar [OPTIONAL]
  Key => data [OPTIONAL]
  ValueLen => uintVar [OPTIONAL]
  Value => data [OPTIONAL]