routing/payment_lifecycle+channeldb: collect existing outcome first

To move towards how we will handle existing attempt in case of MPP
(collecting their outcome will be done in separate goroutines separate
from the payment loop), we move to collect their outcome first.

To easily fetch HTLCs that are still not resolved, we add the utility
method InFlightHTLCs to channeldb.MPPayment.

channeldb/mp_payment.go

@@ -131,6 +131,21 @@ type MPPayment struct {
 	Status PaymentStatus
 }
 
+// InFlightHTLCs returns the HTLCs that are still in-flight, meaning they have
+// not been settled or failed.
+func (m *MPPayment) InFlightHTLCs() []HTLCAttempt {
+	var inflights []HTLCAttempt
+	for _, h := range m.HTLCs {
+		if h.Settle != nil || h.Failure != nil {
+			continue
+		}
+
+		inflights = append(inflights, h)
+	}
+
+	return inflights
+}
+
 // serializeHTLCSettleInfo serializes the details of a settled htlc.
 func serializeHTLCSettleInfo(w io.Writer, s *HTLCSettleInfo) error {
 	if _, err := w.Write(s.Preimage[:]); err != nil {