lib/connections: Don't look at devices that are already optimally connected

Just an optimization. Required exposing the priority from the factory,
so made that an interface with an extra method instead of just a func
type.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3071

lib/connections/structs.go

@@ -28,7 +28,10 @@ type Connection struct {
 	protocol.Connection
 }
 
-type dialerFactory func(*config.Wrapper, *tls.Config) genericDialer
+type dialerFactory interface {
+	New(*config.Wrapper, *tls.Config) genericDialer
+	Priority() int
+}
 
 type genericDialer interface {
 	Dial(protocol.DeviceID, *url.URL) (IntermediateConnection, error)