Implements `stale-while-revalidate` and `s-maxage`

[utix]

Summary

Improve proxy cache plugin to support stale-while-revalidate and s-maxage

Full changelog

stale-while-revalidate

stale-while-revalidate indicates that caches MAY serve the response in which it appears after it becomes stale, up to the indicated number of seconds.

See RFC 5861

s-maxage

s-maxage indicates that, in shared caches, the maximum age specified by this directive overrides the maximum age specified by either:

• the max-age directive or
• the Expires header field.

See RFC 7234

[utix]

@bungle sorry to ping, Should I improve this PR ? Is something missing ?

[hbagdi]

Closing because of lack of activity. We would like to accept this contribution if anyone wishes to pick this back up.