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/domain_settings/domain_setting_edit.php

@@ -307,6 +307,9 @@
 						}
 					}
 
+				//clear domain apcu cache due to changes
+					settings::clear_cache('domain');
+
 				//redirect the browser
 					if ($action == "update") {
 						message::add($text['message-update']);