No need to duplicate this in LibWeb.
In doing so, this also fixes an apparent bug for SWR handling in LibWeb.
We were previously deciding if we were in the SWR lifetime with:
stale_while_revalidate > current_age
However, the SWR lifetime is meant to be an additional time on top of
the freshness lifetime:
freshness_lifetime + stale_while_revalidate > current_age
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 were returning the incorrect result when upgrading a cache entry to
have exclusivity on must-revalidate requests. This could result in the
entry being read and updated at the same time, especially if the server
returned a non-304 response.
There are a couple of remaining RFC 9111 methods in LibWeb's Fetch, but
these are currently directly tied to the way we store GC-allocated HTTP
response objects. So de-coupling that is left as a future exercise.
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.
