This directive allows our disk cache to serve stale responses for a time
indicated by the directive itself, while we revalidate the response in
the background.
Issuing requests that weren't initiated by a client is a new thing for
RequestServer. In this implementation, we associate the request with
the client that initiated the request to the stale cache entry. This
adds a "background request" mode to the Request object, to prevent us
from trying to send any of the revalidation response over IPC.
We currently have two ongoing implementations of RFC 9111, HTTP caching.
In order to consolidate these, this patch moves the implementation from
RequestServer to LibHTTP for re-use within LibWeb.
