Instead of retrying HTTP requests only for 429 status code by default,
prefer to use the generic retry policy enabling to also retry for status
codes >= 500 but also on ConnectionError exceptions.
Rename throttling_retry decorator to http_retry to reflect this change.
In order to get a last_update for each ListedOrigin sent to scheduler
database, send an extra HTTP request for each listed package to the
/api/packages/<package_name> endpoint of pub.dev API.
A pub.dev developer inform us that endpoint is heavily used and cached
so there is no particular issues to query that endpoint for each package
in a row periodically.
