check lightning invoice in-flight

[callebtc]

First draft of an async payment flow. Makes use of previous changes to NUT-07: add new field pending.

Closes #244

Changes

• Adds a new parameter blocking=False to the POST /melt endpoint which makes it return immediately and pay the invoice in the background.
• The wallet then checks for proof states instead of waiting for the endpoint to return as it did before.
• If the proof states indicate that the invoice is paid, wallet goes on as usual

Todo

• store_lightning_invoice only stores the invoice after payment, should store before and update later
• need to come up with a way to check information about the invoice, otherwise we can't get the preimage of a paid invoice. If there should be a new endpoint to check an invoice state, it should be tied to the proofs used to mint for that invoice, otherwise other users can look up the preimage

[AngusP]

Should probably return HTTP 202 Accepted?