Handle Retry-After by default

Let's get some feedback here. This is capped to 30s per retry, so
a maximum of 90s per item that we delay the failure case. In the
worst case, we hence have 90n seconds delay for n items.

In a more realistic worse case however, the server likely does
not collapse per-item, and we end up trying all items in a short
burst so it seems more realistic we hit a 90s total delay.

The bursty behavior may be suboptimal; we don't quite know, but
it seems the specification of Retry-After is per-entity rather than
per-host, so technically speaking it seems correct.

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