local: Limit concurrent backend operations

Use a limit of 2 similar to the filereader concurrency in the archiver.

internal/backend/local/layout_test.go

@@ -37,8 +37,9 @@ func TestLayout(t *testing.T) {
 
 			repo := filepath.Join(path, "repo")
 			be, err := Open(context.TODO(), Config{
-				Path:   repo,
-				Layout: test.layout,
+				Path:        repo,
+				Layout:      test.layout,
+				Connections: 2,
 			})
 			if err != nil {
 				t.Fatal(err)