Co-authored-by: Daniel Norman <[email protected]>

While we are at adding cache-control, we should leverage
stale-while-revalidate hint, especially when we have useful results.

Adding Last-Modified also does not cost us anything, but allows
clients to benefit from caching proxies and CDNs in front of someguy
returning HTTP 304 when request includes If-Modified-Since.

IPNS records should have "string etag" following syntax from
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

this ensures proxies put in front of someguy will use
Accept from request in the cache key, and not mix
application/x-ndjson responses with application/json ones.