(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.
Just thought of a way we might be able to eliminate this one sleep statement.  What if we declare a promise up top, have the ring-handler deliver on the promise in its callback fn, and have the line just after the async/get call wait on the promise to be delivery.  Then, at least, we'd know that by the time the tk-app/stop call were going to be performed that a request had been opened on the Jetty side - assuming it won't take 3 seconds for the promise deref to unblock in conjunction with the connection being closed.  Doesn't eliminate the race conditions completely, though, so probably not worthwhile unless we sporadic failures running these tests later on.