Improve `cache-control` header

[XhmikosR]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control

1. stale-while-revalidate
2. stale-if-error
3. immutable

Maybe not yet, until the spec is in WD phase.

Opening this to track it.

[XhmikosR]

Perhaps we should also remove the ETag header while at it. We already set a proper cache-control header, not sure if the ETag one has any value.

[XhmikosR]

This is something I'd like to sort soon too, mostly stale-while-revalidate.