Bug: Mint tries to invalidate proofs twice in race condition

[callebtc]

Race condition when melting tokens.

• First call: User makes a melt request that takes a long time
• Before the first call returns, user GETs the melt quote
• Melt quote is PAID although the first call hasn't returned yet
• Mint invalidates all proofs of this melt operation
• Now the first call returns, and we try to invalidate the same melt proofs again

Can be reproduced by setting fakewallet_delay_outgoing_payment = 15 to a long time, and fakewallet_payment_state="PAID" (seems to work also for PENDING, probably doesn't matter?).

I think this race condition is very unlikely in a real scenario, because the first call would likely return before any other GET request can race it. Should still be fixed of course.