routing: add new method registerAttempt

This commit adds a new method `registerAttempt` to take care of creating
and saving an htlc attempt to disk.

routing/router.go

@@ -2506,7 +2506,7 @@ func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
 	)
 
 	var shardError error
-	attempt, outcome, err := p.launchShard(rt, false)
+	attempt, outcome, err := p.launchShard(rt, 0)
 
 	// With SendToRoute, it can happen that the route exceeds protocol
 	// constraints. Mark the payment as failed with an internal error.