Do not log out the user when masquerading.
requested to merge issue/simplesamlphp_auth-2124117:2124117-do-not-log-out-when-masquerading into 4.x
This is an alternative patch based on the suggestion in comment https://www.drupal.org/project/simplesamlphp_auth/issues/2124117#comment-13423238
Closes #2124117