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/relay_dial.go

@@ -21,7 +21,7 @@ import (
 const relayPriority = 200
 
 func init() {
-	dialers["relay"] = newRelayDialer
+	dialers["relay"] = relayDialerFactory{}
 }
 
 type relayDialer struct {
@@ -74,9 +74,15 @@ func (d *relayDialer) String() string {
 	return "Relay Dialer"
 }
 
-func newRelayDialer(cfg *config.Wrapper, tlsCfg *tls.Config) genericDialer {
+type relayDialerFactory struct{}
+
+func (relayDialerFactory) New(cfg *config.Wrapper, tlsCfg *tls.Config) genericDialer {
 	return &relayDialer{
 		cfg:    cfg,
 		tlsCfg: tlsCfg,
 	}
 }
+
+func (relayDialerFactory) Priority() int {
+	return relayPriority
+}