Optimize Changelog fetching

[rarkins]

Changelogs often get changes in the first hours after a release but it gets increasingly unlikely over time. It does not make sense for us to check/re-fetch release notes hourly when the release is weeks or months old, etc.

We should look into how we can use timestamps to intelligently increase the caching time of results. Timestamps could come potentially from both (a) the datasource, or (b) the changelogs.