htlcswitch: implement strict forwarding for locally dispatched payments

In this commit, we address an issue that could arise when using the
SendToRoute RPC. In this RPC, we specify the exact hops that a payment
should take. However, within the switch, we would set a constraint for
the first hop to be any hop as long as the first peer was at the end of
it. This would cause discrepancies when attempting to use the RPC as the
payment would actually go through another hop with the same peer. We fix
this by explicitly specifying the channel ID of the first hop.

Fixes #1500.
Fixes #1515.

htlcswitch/test_utils.go

@@ -664,7 +664,7 @@ func (r *paymentResponse) Wait(d time.Duration) (chainhash.Hash, error) {
 // * from Alice to Carol through the Bob
 // * from Alice to some another peer through the Bob
 func (n *threeHopNetwork) makePayment(sendingPeer, receivingPeer lnpeer.Peer,
-	firstHopPub [33]byte, hops []ForwardingInfo,
+	firstHop lnwire.ShortChannelID, hops []ForwardingInfo,
 	invoiceAmt, htlcAmt lnwire.MilliSatoshi,
 	timelock uint32) *paymentResponse {
 
@@ -708,8 +708,9 @@ func (n *threeHopNetwork) makePayment(sendingPeer, receivingPeer lnpeer.Peer,
 
 	// Send payment and expose err channel.
 	go func() {
-		_, err := sender.htlcSwitch.SendHTLC(firstHopPub, htlc,
-			newMockDeobfuscator())
+		_, err := sender.htlcSwitch.SendHTLC(
+			firstHop, htlc, newMockDeobfuscator(),
+		)
 		paymentErr <- err
 	}()