called rapidly by mavproxy

def idle_task(self):
        '''called rapidly by mavproxy'''
        if self.downloaders_lock.acquire(False):
            removed_one = False
            for url in self.downloaders.keys():
                if not self.downloaders[url].is_alive():
                    print("fw: Download thread for (%s) done" % url)
                    del self.downloaders[url]
                    removed_one = True
            if removed_one and not self.downloaders.keys():
                # all downloads finished - parse them
                self.manifests_parse()

            self.downloaders_lock.release()