Caching downloaded feeds on disk
Load a cached feed.
As a convenience, this will also load local feeds.
Check whether feed url is stale.
Returns false if it's stale but last-check-attempt is recent
Low-level part of is_stale that doesn't automatically load the feed overrides (needed to get last_checked).
Useful if you've already loaded them yourself (or confirmed they're missing) to avoid doing it twice.
Touch a 'last-check-attempt' timestamp file for this feed.
This prevents us from repeatedly trying to download a failing feed many
times in a short period.