refund cancelled orders

[bteitelb]

Suggestion by @sgeisler to allow a user to submit an invoice with an order cancellation to receive a refund.

Feedback from @cdecker:

With the plugin implementation that is coming soon we can actually keep the HTLC open until we actually send the message out, at which point we can claim it.
If the user asks for a refund we'll just fail the HTLC without revealing the payment_key. This sort of depends on how long the turnaround is until the message gets sent, since this would lock up funds along the route. Minutes are probably ok, hours not :-)