lib/fs, lib/model, lib/scanner: Make scans cancellable (fixes #3965)

The folder already knew how to stop properly, but the fs.Walk() didn't
and can potentially take a very long time. This adds context support to
Walk and the underlying scanning stuff, and passes in an appropriate
context from above. The stop channel in model.folder is replaced with a
context for this purpose.

To test I added an infiniteFS that represents a large amount of data
(not actually infinite, but close) and verify that walking it is
properly stopped. For that to be implemented smoothly I moved out the
Walk function to it's own type, as typically the implementer of a new
filesystem type might not need or want to reimplement Walk.

It's somewhat tricky to test that this actually works properly on the
actual sendReceiveFolder and so on, as those are started from inside the
model and the filesystem isn't easily pluggable etc. Instead I've tested
that part manually by adding a huge folder and verifying that pause,
resume and reconfig do the right things by looking at debug output.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4117

lib/model/folder.go

@@ -6,14 +6,18 @@
 
 package model
 
-import "time"
+import (
+	"context"
+	"time"
+)
 
 type folder struct {
 	stateTracker
 
 	scan                folderScanner
 	model               *Model
-	stop                chan struct{}
+	ctx                 context.Context
+	cancel              context.CancelFunc
 	initialScanFinished chan struct{}
 }
 
@@ -28,8 +32,9 @@ func (f *folder) Scan(subdirs []string) error {
 	<-f.initialScanFinished
 	return f.scan.Scan(subdirs)
 }
+
 func (f *folder) Stop() {
-	close(f.stop)
+	f.cancel()
 }
 
 func (f *folder) Jobs() ([]string, []string) {
@@ -39,7 +44,7 @@ func (f *folder) Jobs() ([]string, []string) {
 func (f *folder) BringToFront(string) {}
 
 func (f *folder) scanSubdirs(subDirs []string) error {
-	if err := f.model.internalScanFolderSubdirs(f.folderID, subDirs); err != nil {
+	if err := f.model.internalScanFolderSubdirs(f.ctx, f.folderID, subDirs); err != nil {
 		// Potentially sets the error twice, once in the scanner just
 		// by doing a check, and once here, if the error returned is
 		// the same one as returned by CheckFolderHealth, though