introduce reopen() method to be used for unit test execution only - right after a unit test has been executed the session will be reopened

adding an explicit close method to class session - write operations (set and remove) being called after close() will throw an exception

PHP Fatal error: Can't inherit abstract function OCP\ISession::set() (previously declared abstract in OC\Session\Session)

Conflicts:
	lib/private/vcategories.php