pull: Download and checksum asynchronously

This is quite a noticeable speedup when downloading loose objects.

src/libotutil/ot-gio-utils.h

@@ -88,6 +88,16 @@ gboolean ot_gio_checksum_stream (GInputStream   *in,
                                  GCancellable   *cancellable,
                                  GError        **error);
 
+void ot_gio_checksum_stream_async (GInputStream         *in,
+                                   int                   io_priority,
+                                   GCancellable         *cancellable,
+                                   GAsyncReadyCallback   callback,
+                                   gpointer              user_data);
+
+guchar * ot_gio_checksum_stream_finish (GInputStream   *in,
+                                        GAsyncResult   *result,
+                                        GError        **error);
+
 gboolean ot_gfile_merge_dirs (GFile    *destination,
                               GFile    *src,
                               GCancellable *cancellable,