Adjust segmentation spec

[Ivansete-status]

All follow-ups listed in #332 are addressed.

Besides:

• Wire-format-first structure. It's been some additions and removals to SegmentMessage.

  message SegmentMessage {
     bytes  entire_message_hash  = 1;  // Keccak256 of the original payload, 32 bytes
     uint32 index                = 2;  // position within this segment's own class (data or parity)
     uint32 segment_count        = 3;  // size of this segment's own class (data or parity)
     bool   is_parity            = 4;  // selects the class the two fields above refer to
     bytes  payload              = 5;  // data chunk or parity shard
     uint32 payload_length       = 6;  // length in bytes of the original payload
  }

closes #332