[Mint] Synthetic USD E-Cash with LNMarketsWallet

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Synthetic USD (s-USD) on LNMarkets is achieved by shorting BTCUSD futures with a leverage equal to 1. LNMarketsWallet mints USD ecash by querying bolt11 invoices for s-USD deposits of the equivalent value in msat. Naturally, these invoices have a short expiry (120) because the exchange rate is volatile. Unlike StrikeWallet, this approach removes the mint operator's necessity of having to register to a KYC platform, as there is no interaction with Trad-Fi. LNMarkets is a startup, but their platform is open source and anybody could potentially start an exchange.

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This needs better error messages...