Split CacheMiddleware up into two parts -- an update-cache and a...

Split CacheMiddleware up into two parts -- an update-cache and a fetch-from-cache middleware. This lets you run each half of the cache middleware at the correct time to avoid bad interactions between the cache middleware and other middleware that must modify the cache key (like the locale middleware).

CacheMiddleware itself is still around for backwards-compatibility and as a hook point for the cache decorator, but the documentation has been updated to point people towards the two-part caching middleware.

Refs #730.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8260 bcc190cf-cafb-0310-a4f2-bffc1f526a37