code: func (s *sentDownloadState) cleanup(folder string) []protocol.FileDownloadProgressUpdate {
  	fs, ok := s.folderStates[folder]
  	if ok {
  		updates := fs.destroy()
  		delete(s.folderStates, folder)
  		return updates
  	}
  	return nil
  }
docs: // cleanup cleans up all state related to a folder, and returns a set of updates
  // which would clean up the state on the remote device.