routing/payment_session: make NewPaymentSession take payment directly

This commit moves supplying of the information in the LightningPayment
to the initialization of the paymentSession, away from every call to
RequestRoute.

Instead the paymentSession will store this information internally, as it
doesn't change between payment attempts.

This is done to rid the RequestRoute call of the LightingPayment
argument, as for SendToRoute calls, it is not needed to supply the next
route.

routing/router.go

@@ -159,8 +159,7 @@ type PaymentSessionSource interface {
 	// routes to the given target. An optional set of routing hints can be
 	// provided in order to populate additional edges to explore when
 	// finding a path to the payment's destination.
-	NewPaymentSession(routeHints [][]zpay32.HopHint,
-		target route.Vertex) (PaymentSession, error)
+	NewPaymentSession(p *LightningPayment) (PaymentSession, error)
 
 	// NewPaymentSessionForRoute creates a new paymentSession instance that
 	// is just used for failure reporting to missioncontrol, and will only
@@ -1677,9 +1676,7 @@ func (r *ChannelRouter) preparePayment(payment *LightningPayment) (
 	// Before starting the HTLC routing attempt, we'll create a fresh
 	// payment session which will report our errors back to mission
 	// control.
-	paySession, err := r.cfg.SessionSource.NewPaymentSession(
-		payment.RouteHints, payment.Target,
-	)
+	paySession, err := r.cfg.SessionSource.NewPaymentSession(payment)
 	if err != nil {
 		return nil, err
 	}
@@ -1813,14 +1810,13 @@ func (r *ChannelRouter) sendPayment(
 	// Now set up a paymentLifecycle struct with these params, such that we
 	// can resume the payment from the current state.
 	p := &paymentLifecycle{
-		router:         r,
-		payment:        payment,
-		paySession:     paySession,
-		currentHeight:  currentHeight,
-		finalCLTVDelta: uint16(payment.FinalCLTVDelta),
-		attempt:        existingAttempt,
-		circuit:        nil,
-		lastError:      nil,
+		router:        r,
+		payment:       payment,
+		paySession:    paySession,
+		currentHeight: currentHeight,
+		attempt:       existingAttempt,
+		circuit:       nil,
+		lastError:     nil,
 	}
 
 	// If a timeout is specified, create a timeout channel. If no timeout is