Isolates the __page_lookup cache from other projects using the same cache backend

I was noticing an issue where the root page '/' would resolve to '/en/' even when the project had USE_I18N=False and the languageCookie MW disabled.

Turns out that this is because the __page_lookup cache uses cache-keys that are not namespaced to the project.  This was only happening on my production server, which incidentally has many other projects on, all using memcached.  The root page '/', just happened to be the one to collide, as the projects are less likely to have other pages with the same name.

This should be back-ported to 2.4. -- Will follow-up with another PR for 2.4