Introduce `wp_using_ext_object_cache()` - mimic `wp_suspend_cache_invalidation()` and discourage direct access to `$_wp_using_ext_object_cache`, cleaning up importing of globals in functions and provides function to modify that global.  Loads the packaged object cache when an external cache hasn't been loaded or doesn't contain `wp_cache_init()`.

Fixes #21401.



git-svn-id: https://develop.svn.wordpress.org/trunk@25289 602fd350-edb4-49c9-b593-d223f7449a82