Speed up xbmc ui nav by change loaders use StopAsync(true)

currently, each time when nav win changed the path, the bg info loader has to wait all workers stopped and block ui. if the worker is working on a slow task, e.g. list a zip file or on slow link, this will make the ui looks 'lock'.
This PR changed the StopAsync method of bg loader, make it support finish (save) current work asap, before the working workers finish their half work.