[Nuts] Notes: Async LN payment endpoint design

[callebtc]

Notes to myself for implementing async LN payments after we have this nut PR merged: cashubtc/nuts#21

• an async /melt endpoint would take in proofs and a LN payment destination
• the endpoint immediately returns a random checking ID (not payment hash!)
• the wallet then repeatedly asks for the payment state including blank outputs for overpaid fees
• if the invoice has been paid, the mint marks it as such and includes the paid fees
• if the invoice is paid, and the wallet asks for the payment state, the mint signs the included blank outputs to return fees
• when the fees are returned, the mint marks the invoice as "fees_returned"