multi: do not use tlv.Record outside wire format handling

This commit prepares for more manipulation of custom records. A list of
tlv.Record types is more difficult to use than the more basic
map[uint64][]byte.

Furthermore fields and variables are renamed to make them more
consistent.

routing/payment_session.go

@@ -129,7 +129,7 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	sourceVertex := route.Vertex(ss.SelfNode.PubKeyBytes)
 	route, err := newRoute(
 		payment.Amount, sourceVertex, path, height, finalCltvDelta,
-		payment.FinalDestRecords,
+		payment.DestCustomRecords,
 	)
 	if err != nil {
 		// TODO(roasbeef): return which edge/vertex didn't work