Fixed #14596 -- Light refactoring of the cache backends.

 * Removes some code duplication,
 * Provides a convenient base class for db-like cache backends
 * Adds tests for an edge case of culling,
 * Marks the memcached tests as "skipped", rather than omitting them.

Thanks to Jonas H for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14434 bcc190cf-cafb-0310-a4f2-bffc1f526a37