Feat: improve thumbnails proformance and GC for local policy (#1044)

* thumb generating improvement

Replace "github.com/nfnt/resize" with "golang.org/x/image/draw". Add thumb task queue to avoid oom when batch thumb operation

* thumb improvement

* Add some tests for thumbnail generation

pkg/filesystem/image_test.go

@@ -38,3 +38,12 @@ func TestFileSystem_GetThumb(t *testing.T) {
 		asserts.EqualValues(50, res.MaxAge)
 	}
 }
+
+func TestFileSystem_ThumbWorker(t *testing.T) {
+	asserts := assert.New(t)
+
+	asserts.NotPanics(func() {
+		getThumbWorker().addWorker()
+		getThumbWorker().releaseWorker()
+	})
+}