Previously, a race between the server starting to reject requests
on a kept-alive connection and the request reaching the blocking
servlet could result in a response never being sent.

This commit updates the test to disable blocking once graceful
shutdown with an in-flight request has being. Awaitility is then used
to make a request on an idle kept-alive connection until it fails
due to the connection reset. This may not happen immediately due to
the aforementioned race.