[chore] Do cache-control in a less silly way to avoid writing header twice

[tsmethurst]

Description

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I was way too hasty merging #1480.

This PR achieves the same effect, but without having to write the Cache-Control header twice for every request when using non-proxying s3; instead the storage method is just checked once at startup and the appropriate cache-control timeout is selected then.

Checklist

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