[Mint] Check "already signed" output for blank outputs in /melt

[callebtc]

We check whether outputs have already been signed for a /swap and /mint but not for the change outputs for a /melt.

Edit: checking already signed outputs might not be enough since two requests with the same outputs can both turn out to be unsigned yet but still cause a race condition. A real fix needs a lock on the outputs like we do it for pending proofs.