routing: hop-payload for last hop should be the absolute timeout, not delta

This commit fixes an oversight in the path finding code when converting
a path into a route. Currently, for the last hop, we’d emplace the
expiry delta of the last hop within the per-hop payload. This was left
over from a prior version of the specification.

To fix this, we’ll now emplace the _absolute_ final HTLC expiry with
the payload, such that, the final hop that verify that the HTLC has not
been tampered with in flight.

routing/router.go

@@ -1002,6 +1002,7 @@ func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte, *Route
 	// payment amount. If no such routes can be found then an error will be
 	// returned.
 	if !ok {
+		// TODO(roasbeef): put cache handling into FindRoutes
 		freshRoutes, err := r.FindRoutes(payment.Target, payment.Amount)
 		if err != nil {
 			return preImage, nil, err