Add the apcu caching ability for performance (#7276)

* add the apcu caching ability for performance
When the PHP extension APCu is loaded, the settings class and the auto_loader will cache their results across requests in RAM. For more information about the APCu extension visit the PHP page: https://www.php.net/apcu

* use global instead of default terminology

core/default_settings/default_settings_reload.php

@@ -46,7 +46,7 @@ $search = $_REQUEST['search'] ?? '';
 $domain_uuid = $_GET['id'] ?? null;
 
 //reload default settings
-require "resources/classes/domains.php";
+settings::clear_cache();
 $domain = new domains();
 $domain->set();