Support for changing the Cache-Control header sent by the GET API

[mildbyte]

(followup to https://github.com/splitgraph/seafowl/issues/20)

The current default is not sending any cache-control headers at all, which means that the browser/CDNs will lean towards more caching. (from my basic testing, it seems like Cloudflare caches for 2h at least: https://developers.cloudflare.com/cache/about/edge-browser-cache-ttl/)

Possible settings:

• how soon to revalidate with an If-None-Match (maxage?)
• sending s-maxage as well as maxage (the former seems to influence CDNs)

Some reference: https://jakearchibald.com/2016/caching-best-practices/