archiver: unify FutureTree/File into futureNode

There is no real difference between the FutureTree and FutureFile
structs. However, differentiating both increases the size of the
FutureNode struct.

The FutureNode struct is now only 16 bytes large on 64bit platforms.
That way is has a very low overhead if the corresponding file/directory
was not processed yet.

There is a special case for nodes that were reused from the parent
snapshot, as a go channel seems to have 96 bytes overhead which would
result in a memory usage regression.

internal/archiver/file_saver_test.go

@@ -64,7 +64,7 @@ func TestFileSaver(t *testing.T) {
 	testFs := fs.Local{}
 	s, ctx, wg := startFileSaver(ctx, t)
 
-	var results []FutureFile
+	var results []FutureNode
 
 	for _, filename := range files {
 		f, err := testFs.Open(filename)
@@ -77,14 +77,14 @@ func TestFileSaver(t *testing.T) {
 			t.Fatal(err)
 		}
 
-		ff := s.Save(ctx, filename, f, fi, startFn, completeFn)
+		ff := s.Save(ctx, filename, filename, f, fi, startFn, completeFn)
 		results = append(results, ff)
 	}
 
 	for _, file := range results {
-		file.Wait(ctx)
-		if file.Err() != nil {
-			t.Errorf("unable to save file: %v", file.Err())
+		fnr := file.take(ctx)
+		if fnr.err != nil {
+			t.Errorf("unable to save file: %v", fnr.err)
 		}
 	}