channeldb+routing: apply method Terminated to decide a payment's terminal state

This commit applies the new method `Terminated`. A side effect from
using this method is, we can now save one query `fetchPayment` inside
`FetchInFlightPayments`.

channeldb/payment_control.go

@@ -722,21 +722,16 @@ func (p *PaymentControl) FetchInFlightPayments() ([]*MPPayment, error) {
 				return fmt.Errorf("non bucket element")
 			}
 
-			// If the status is not InFlight, we can return early.
-			paymentStatus, err := fetchPaymentStatus(bucket)
-			if err != nil {
-				return err
-			}
-
-			if paymentStatus != StatusInFlight {
-				return nil
-			}
-
 			p, err := fetchPayment(bucket)
 			if err != nil {
 				return err
 			}
 
+			// Skip the payment if it's terminated.
+			if p.Terminated() {
+				return nil
+			}
+
 			inFlights = append(inFlights, p)
 			return nil
 		})