Using the policy change signal from QNetworkSession.
If the new policy disallows background requests and this is a
background request, then generate an error.
This results in a TCP RST on the socket, and a
BackgroundRequestNotAllowedError on the QNetworkReply.
If the reply is already finished, no action is taken.
Change-Id: I4ff5c681a8b7b852727bb95f03664d666f4efe07
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>