UrlStore: add blocking convenience function around get_download_urls()

[adbar]

In order to facilitate the generation of download lists, add a function to the store along those lines: repeated calls to get_download_urls() until there are no more URLs to download.

while True:
    bufferlist = self.get_download_urls()
    if bufferlist or self.done:
        break
    sleep(sleep_time)
return bufferlist