Ensure that seeds are refetched from time to time even if failed or redirected

[sebastian-nagel]

All manually collected seeds (feeds or future seed formats, e.g., news sitemaps) should be refetched from time to time (weekly or monthly) even if they are redirected or failed to fetch:

• a redirect may change over time. This indeed happened within 2 month for 10 seeds out of 5000: 8 redirects now point to https instead of http, two redirects have changed more significantly
• any failure (failed to fetch, failed to parse, etc.) which is potentially only a transient error

Ideally, the fetch schedule should be configurable for a combination of metadata and fetch status.

[sebastian-nagel]

Testing DigitalPebble/storm-crawler#386 in production, waiting for DigitalPebble/storm-crawler#420