Loosen Cache-control header

[joepio]

Currently, AtomicServer returns: Cache-Control: no-store, no-cache, must-revalidate, private.

This is perhaps a tad too restrictive, it slows down apps unnecessarily

[joepio]

So there is a reason for this exissting no-cache stuff: #137

In short: the the browser caches json responses and shows these instead of the html page.

Also, this header is probably not that bad, as it only affects resource routes.

The /download endpoint seems to give the correct etag etc: