BUG/MEDIUM: http: Close streams for connections closed before a redirect

BUG/MEDIUM: http: Close streams for connections closed before a redirect

A previous fix was made to prevent the connection to a server if a redirect was
performed during the request processing when we wait to keep the client
connection alive. This fix introduced a pernicious bug. If a client closes its
connection immediately after sending a request, it is possible to keep stream
alive infinitely. This happens when the connection closure is caught when the
request is received, before the request parsing.

To be more specific, this happens because the close event is not "forwarded",
first because of the call to "channel_dont_connect" in the function
"http_apply_redirect_rule", then because we want to keep the client connection
alive, we explicitly call "channel_dont_close" in the function
"http_request_forward_body".

So, to fix the bug, instead of blocking the server connection, we force its
shutdown. This will force the stream to re-evaluate all connexions states. So it
will detect the client has closed its connection.

This patch must be backported in 1.7.