Add support for automatic template based xkey

Add support for automatic template based xkey

This adds an xkey header to all outgoing requests with the hash of the
name of all templates loaded. In the future we will be able to use this
to purge "all pages that included a specific template", regardless of
where in the hierarchy it was loaded.

Do this by faking a template loader that never finds anything -- but it
will record the path of the template that it tried to load, and then
leave it to another template loader to actually load it. Store this in
thread local storage (it's a bit ugly, but it's the only thing Django
supports for storing things at the request level from a template
loader), and fetch it from the middleware.