message: Update the double-spend-proof-specification
  Indicate that we have DoubleSpendA and DoubleSpendB rather
  than FirstSpender and SecondSpender,  since
  we can't know which one actually came first.
diff: @@ -84,8 +84,8 @@ cause a reply with the following message. The name of the message is **`dsproof-
  | -----------|:-----------:| ----------:|---------:|
  | 32 | TxInPrevHash | sha256 | The txid being spent |
  | 4 | TxInPrevIndex | int | The output being spent |
  -| | FirstSpender | spender | the first sorted spender |
  -| | DoubleSpender | spender | the second spender |
  +| | DoubleSpendA | spender | the first sorted spender |
  +| | DoubleSpendB | spender | the second spender |
  A double-spend-proof essentially describes two inputs, both spending the
  same output. As such the prev-hash and prev-index point to the output and
  