Automatically refresh outdated data in collections

From the milestone 2 requirements:

SHOULD: Enhance mechanism that updates a collection with a way to refresh outdated data (data older than X days) by rescheduling the associated tasks.

One important use case is the way that the Debian archive is reflected in debusine: in order to run automated QA tasks against the archive, debusine needs to periodically refresh its collections that represent the archive.