Allow override of default clean shutdown behavior

By default, if a web-app is declared <distributable/>, we will only wait until there are no pending requests and all sessions are safely replicated. Only if a web-app is not <distributable/>, do we wait until there are no more active sessions, since there is no session replication. We need the ability to allow the user to override this default behavior, and use session draining even for <distributable/> apps, to avoid overloading remaining servers with failover requests.