439895 - No event callback should be invoked after the "failure" callback.

Fixed HttpSender and HttpReceiver to use a non-blocking collaborative
mechanism to notify callbacks.
Only the "failed" callback can run concurrently with other callbacks.
No other callback can run after the "complete" callback: a failure
concurrent with another callback will notify the "failed" callback,
finish the running callback and only then invoke the "complete" callback.
440020 - ProxyServlet does not handle correctly failure after committed response to client.

Fixed by introducing a request attribute "org.eclipse.jetty.server
.Response.failure" used by HttpChannel to immediately close the
connection when it sees it.