wp_using_ext_object_cache

切换`$_wp_using_ext_object_cache`的开与关，而不直接接触全局。

function wp_using_ext_object_cache( $using = null ) {
	global $_wp_using_ext_object_cache;
	$current_using = $_wp_using_ext_object_cache;
	if ( null !== $using ) {
		$_wp_using_ext_object_cache = $using;
	}
	return $current_using;
}