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/pathfind.go

@@ -274,10 +274,10 @@ func newRoute(amtToSend lnwire.MilliSatoshi, pathEdges []*ChannelHop,
 		route.TotalTimeLock += uint32(edge.TimeLockDelta)
 
 		// If this is the last hop, then for verification purposes, the
-		// value of the outgoing time-lock should be _exactly_ the time
-		// lock delta specified within the routing information.
+		// value of the outgoing time-lock should be _exactly_ the
+		// absolute time out they'd expect in the HTLC.
 		if i == len(pathEdges)-1 {
-			nextHop.OutgoingTimeLock = uint32(edge.TimeLockDelta)
+			nextHop.OutgoingTimeLock = currentHeight + uint32(edge.TimeLockDelta)
 		} else {
 			// Otherwise, the value of the outgoing time-lock will
 			// be the value of the time-lock for the _outgoing_

routing/pathfind_test.go

@@ -382,7 +382,7 @@ func TestBasicGraphPathFinding(t *testing.T) {
 		t.Fatalf("expected outgoing time-lock of %v, instead have %v",
 			1, route.Hops[0].OutgoingTimeLock)
 	}
-	if route.Hops[1].OutgoingTimeLock != 1 {
+	if route.Hops[1].OutgoingTimeLock != 101 {
 		t.Fatalf("outgoing time-lock for final hop is incorrect: "+
 			"expected %v, got %v", 1, route.Hops[1].OutgoingTimeLock)
 	}

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