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.
440038 - Content decoding may fail.

Properly looping around the decoding step to ensure that the encoded
content is fully consumed.
439788 - CORS filter headers gone between 9.2.0.M0 and 9.2.1 .v20140609 for ProxyServlet requests.

Removed the offending code that was added only to avoid duplicate
Date headers, but sending of those headers is configurable from
HttpConfiguration.
411323 - DosFilter/QoSFilter should use AsyncContext rather than Continuations.
440122 - Remove usages of ForkInvoker.
Restored IteratingCallback APIs (in particular the completed()
method) to keep compatibility over micro versions of Jetty.

Removed getState() - can't return a private class from a protected
method, plus it was only used in a toString() with the wrong
formatting string, that was already printing the state.

Removed also final modifiers to keep compatibility.
Reorganized GZIP tests.
Merged branch 'master' into 'jetty-http2'.