Refactor config commit stuff to support restartless updates better

Includes restartless updates of the GUI settings (listening port etc) as
a proof of concept.

internal/config/config.go

@@ -20,14 +20,11 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/calmh/logger"
 	"github.com/syncthing/protocol"
 	"github.com/syncthing/syncthing/internal/osutil"
 	"golang.org/x/crypto/bcrypt"
 )
 
-var l = logger.DefaultLogger
-
 const (
 	OldestHandledVersion = 5
 	CurrentVersion       = 10