MDL-46099 session: fix use of references for session globals

This reverses the references used for global $USER and $SESSION,
the reason is that PHP does not allow references to references.
$USER is a reference to $GLOBALS['USER'] which means we cannot
put any references to it. Solution is to store the current user and session
objects in $GLOBALS['USER'] and $GLOBALS['SESSIOn'] are reference
them in $_SESSION.

This patch makes the session code behave the same way in CLI,
phpunit and normal web requests - this allows use to finally
unit test most aspects of the session code in Moodle.

lib/setup.php

@@ -773,10 +773,6 @@ if (empty($CFG->sessiontimeout)) {
     $CFG->sessiontimeout = 7200;
 }
 \core\session\manager::start();
-if (!PHPUNIT_TEST and !defined('BEHAT_TEST')) {
-    $SESSION =& $_SESSION['SESSION'];
-    $USER    =& $_SESSION['USER'];
-}
 
 // Initialise some variables that are supposed to be set in config.php only.
 if (!isset($CFG->filelifetime)) {