Removed unused field in ResponseNotifier.
Catching Throwable in RequestNotifier and ResponseNotifier when
notifying listeners.
430808 - OutputStreamContentProvider violates OutputStream contract.

Fixed OutputStreamContentProvider to perform blocking write() calls and
implemented OutputStream.flush().
HttpSender now notifies the ContentProvider iterator of write completion
and if it implements Callback.
This is used in DeferredContentProvider to provide a blocking flush()
functionality.
424107 - Jetty should not finish chunked encoding on exception.

Implemented abort() - empty - in HttpTransportOverFastCGI.