On systems with slow disks, the recursive scanning of directories can be expensive -- it takes upwards of 2 minutes on our systems. This can block the main loop for such a long time that it allows the download to time out... As such, move all the scanning of objects to a queue, processed from an idle, to make sure that we don't block the main loop when scanning. https://bugzilla.gnome.org/show_bug.cgi?id=753336