Implement stale-while-revalidate and stale-if-error cache control directives

[RobinHoutevelts]

Summary

stale-if-error gives you the control over how a CDN should behave in case of errors. stale-while-revalidate can improve the perceived performance of your website, if your website doesn't require absolute data freshness.

It would be nice to use these directives for improved CDN performance.

• https://aws.amazon.com/about-aws/whats-new/2023/05/amazon-cloudfront-stale-while-revalidate-stale-if-error-cache-control-directives/
• https://www.fastly.com/blog/stale-while-revalidate-stale-if-error-available-today
• https://www.keycdn.com/blog/keycdn-supports-stale-while-revalidate

[RobinHoutevelts]

Done in https://github.com/wieni/wmcontroller/pull/61