(TK-92) Add graceful-shutdown support

Add graceful-shutdown support to the Jetty9 Webservice. Now,
when the webserver is shutdown, the server will give open
requests a chance to resolve themselves. Allow user to
configure the graceful shutdown stop timeout.
The simplification works for me.  I think it would be good to make a `.setStopTimeout s 0` call even for a case where we don't register the StatisticsHandler.  That would keep any other Servlets that happen to implement `Graceful` in synch with our intention to timeout immediately.