Fix EZP-23176: Sessions are always started for anonymous user

JIRA: https://jira.ez.no/browse/EZP-23176

`eZUser::setCurrentlyLoggedInUser()` always starts a session, even for anonymous.

Replaces ezsystems/ezpublish-kernel#932

Essentially this will not start a new session if the (new) current user is anonymous.

If a previously user was set, and setCurrentlyLoggedInUser() is for anonymous, a session should already exist, so it will still update the user ID.