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.
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.
