all: Simultaneously walk fs and db on scan (fixes #2571, fixes #4573) (#4584)

When scanner.Walk detects a change, it now returns the new file info as well as the old file info. It also finds deleted and ignored files while scanning.
Also directory deletions are now always committed to db after their children to prevent temporary failure on remote due to non-empty directory.

lib/fs/walkfs.go

@@ -10,7 +10,10 @@
 
 package fs
 
-import "path/filepath"
+import (
+	"path/filepath"
+	"sort"
+)
 
 // WalkFunc is the type of the function called for each file or directory
 // visited by Walk. The path argument contains the argument to Walk as a
@@ -54,6 +57,7 @@ func (f *walkFilesystem) walk(path string, info FileInfo, walkFn WalkFunc) error
 	if err != nil {
 		return walkFn(path, info, err)
 	}
+	sort.Strings(names)
 
 	for _, name := range names {
 		filename := filepath.Join(path, name)