Add class CDBCloseGuard for auto-close database when leave scope.
Add exit points for loaders' LoadItem() to speed up if thread going to stop.
Change CBackgroundInfoLoader::StopAsync() to support finish and discard left work.
Change CBackgroundInfoLoader::Load() to stop previous work asap.
Change all CBackgroundInfoLoader::StopThread() to StopAsync(true) to fasten xbmc ui.