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/user_settings/user_setting_edit.php

@@ -298,6 +298,9 @@ if (!empty($_POST) && empty($_POST["persistformvar"])) {
 					}
 				}
 
+			//clear the user settings cache
+				settings::clear_cache('user');
+
 			//redirect the browser
 				if ($action == "update") {
 					message::add($text['message-update']);