Add warning when using cache keys that might not work with memcached.

This means testing with local dev caches (not memcache) will warn
developers if they are introducing inadvertent importabilities. There is
also the ability to silence the warning if a dev is not planning to use
memcache and knows what they are doing with their keys.

Thanks to Carl Meyer for the patch. Fixed #6447.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13766 bcc190cf-cafb-0310-a4f2-bffc1f526a37