Introduce wp_get_db_schema() for rerieving the various flavors of the WP db...

Introduce wp_get_db_schema() for rerieving the various flavors of the WP db schema.  Eliminates need to use  global. Allows multiple calls to wpmu_create_blog(). see #12028

git-svn-id: https://core.svn.wordpress.org/trunk@18899 1a063a9b-81f0-0310-95a4-ce76da25c4cd