routing: always update payment in the same goroutine

This commit refactors `collectResultAsync` such that this method is now
only responsible for collecting results from the switch. The method
`decideNextStep` is expanded to process these results in the same
goroutine where we fetch the payment from db, to make sure the lifecycle
loop always have a consistent view of a given payment.

routing/router.go

@@ -1186,7 +1186,7 @@ func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
 
 	// The attempt was successfully sent, wait for the result to be
 	// available.
-	result, err = p.collectResult(attempt)
+	result, err = p.collectAndHandleResult(attempt)
 	if err != nil {
 		return nil, err
 	}