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();
 

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']);

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']);