(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.
Do we need the separate thread for performing the stop?  If `async/get` handles getting the response on a separate thread anyway, maybe this could just be changed to doing the following on a single thread:

* response (async/get ...
* tk-app/stop
* check that Jetty was stopped (see the `jetty-and-dependent-service-shutdown-after-service-error` test in 'jetty9_server_test.clj' for an example of how we've done that elsewhere
* deref response and check result to make sure it has good values