This ServletHolder's sole purpose is to provide the thread's current
ServletHolder for realms that are interested in the current servlet, e.g.
current servlet name.
It is also being our servlet gbean for now. We could gbean-ize the superclass to avoid the thread local access.
version: $Rev: 609448 $ $Date: 2008-01-06 14:31:09 -0800 (Sun, 06 Jan 2008) $ See Also:JAASJettyRealm.isUserInRole(java.security.PrincipalString)