Code clean up in session pool

(1) Unnecessary checking if session is PooledSession

if (session instanceof PooledSession)

{ session.logout(); }

else {
// for impersonated non-pooled session
if (session.isLive())

{ session.logout(); }

}
(2) session.isLive() should be blocked with try catch because it could throw unexpected RMI runtime exception.
(3) refresh() method of PooledSession should be allowed only in activated status.
(4) Refine comments.