lib/scanner: Refactor scanner.Walk API

The old usage pattern was to create a Walker with a bunch of attributes,
then call Walk() on it and nothing else. This extracts the attributes
into a Config struct and exposes a Walk(cfg Config) method instead, as
there was no reason to expose the state-holding walker type.

Also creates a few no-op implementations of the necessary interfaces
so that we can skip nil checks and simiplify things here and there.

Definitely look at this diff without whitespace.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3060

lib/model/model.go

@@ -1401,7 +1401,9 @@ func (m *Model) internalScanFolderSubdirs(folder string, subs []string) error {
 	cancel := make(chan struct{})
 	defer close(cancel)
 
-	w := &scanner.Walker{
+	runner.setState(FolderScanning)
+
+	fchan, err := scanner.Walk(scanner.Config{
 		Folder:                folderCfg.ID,
 		Dir:                   folderCfg.Path(),
 		Subs:                  subs,
@@ -1417,11 +1419,8 @@ func (m *Model) internalScanFolderSubdirs(folder string, subs []string) error {
 		ShortID:               m.shortID,
 		ProgressTickIntervalS: folderCfg.ScanProgressIntervalS,
 		Cancel:                cancel,
-	}
+	})
 
-	runner.setState(FolderScanning)
-
-	fchan, err := w.Walk()
 	if err != nil {
 		// The error we get here is likely an OS level error, which might not be
 		// as readable as our health check errors. Check if we can get a health