routing: add new FinalCLTVDelta attribute to SendPayment

In this commit, we’ll now optionally allow the user to pass in the CLTV
delta value specified by the recipient a payment. If the value isn’t
specified, then we’ll use the current global default for the payment.

routing/router.go

@@ -1164,6 +1164,13 @@ type LightningPayment struct {
 	// the first hop.
 	PaymentHash [32]byte
 
+	// FinalCLTVDelta is the CTLV expiry delta to use for the _final_ hop
+	// in the route. This means that the final hop will have a CLTV delta
+	// of at least: currentHeight + FinalCLTVDelta. If this value is
+	// unspcified, then a default value of DefaultFinalCLTVDelta will be
+	// used.
+	FinalCLTVDelta *uint16
+
 	// TODO(roasbeef): add e2e message?
 }
 
@@ -1194,6 +1201,13 @@ func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte, *Route
 		return preImage, nil, err
 	}
 
+	var finalCLTVDelta uint16
+	if payment.FinalCLTVDelta == nil {
+		finalCLTVDelta = DefaultFinalCLTVDelta
+	} else {
+		finalCLTVDelta = *payment.FinalCLTVDelta
+	}
+
 	// We'll continue until either our payment succeeds, or we encounter a
 	// critical error during path finding.
 	for {