* Introduce clean_blog_cache() so we can run it independently of refresh_blog_details() which assumes the blog still exists and get_blog_details() can be called.
* Don't db escape cache keys in get_blog_id_from_url()
* prepare() the query in get_blog_id_from_url()
* Return 0 for all failures in get_blog_id_from_url()
* clean_blog_cache() after dropping tables in wpmu_delete_blog() to make sure the cache is for real cleaned.
git-svn-id: http://core.svn.wordpress.org/trunk@22092 1a063a9b-81f0-0310-95a4-ce76da25c4cd