routing+routerrpc: notify full payment structures

This commit fixes the inconsistency between the payment state as
reported by routerrpc.SendPayment/routerrpc.TrackPayment and the main
rpc ListPayments call.

In addition to that, payment state changes are now sent out for every
state change. This opens the door to user interfaces giving more
feedback to the user about the payment process. This is especially
interesting for multi-part payments.

routing/mock_test.go

@@ -457,7 +457,7 @@ func (m *mockControlTower) FetchInFlightPayments() (
 }
 
 func (m *mockControlTower) SubscribePayment(paymentHash lntypes.Hash) (
-	bool, chan PaymentResult, error) {
+	*ControlTowerSubscriber, error) {
 
-	return false, nil, errors.New("not implemented")
+	return nil, errors.New("not implemented")
 }