Add check for unused WebSocket sessions

Sessions connected to a STOMP endpoint are expected to receive some
client messages. Having received none after successfully connecting
could be an indication of proxy or network issue. This change adds
periodic checks to see if we have not received any messages on a
session which is an indication the session isn't going anywhere
most likely due to a proxy issue (or unreliable network) and close
those sessions.

Issue: SPR-11884
Improve JettySockJsIntegrationTests to show real error

In a recent CI build failure a test timed out waiting for a message:
https://build.spring.io/browse/SPR-PUB-1316

In fact there was a WebSocket transport failure originating in Jetty's
WebSocket Parser. However this failure is only apparent by looking at
the logs. This change adds a check if a transport error ocurred while
we were waiting and throws an AssertionError.