refund cancelled orders

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 :-)