[1.2.X] 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.

Backport of r13766 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13767 bcc190cf-cafb-0310-a4f2-bffc1f526a37