So far we were relying on `lightningd` to create an ad-hoc invoice when
telling it to `resolve` with a given preimage. We now switch to having the
plugin create the invoice, remove the mandatory `keysend_preimage`
field (which would upset `lightningd` otherwise), and then return the modified
payload with the instructions to `continue` instead of resolving.
This ties back in with the existing payment/invoice handling code. Invoices
are created only if we don't have a label clash (unlikely since we have the
nano-time in the label), or the `payment_hash` was already used for another
invoice (at which point `lightningd` will automatically reject the payment and
we're a bit poorer for it, but meh :-)
This still uses the experimental TLV-type, but once the type is standardized
we can add detection for the new type quite easily.
Changelog-Added: pay: The `keysend` plugin implements the ability to receive spontaneous payments (keysend)
Christian Decker