BUG/MEDIUM: http-ana: Report 502 from req analyzer only during rsp forwarding

BUG/MEDIUM: http-ana: Report 502 from req analyzer only during rsp forwarding

A server abort must be handled by the request analyzers only when the
response forwarding was already started. Otherwise, it it the responsability
of the response analyzer to detect this event. L7-retires and conditions to
decide to silently close a client conneciotn are handled by this analyzer.

Because a reused server connections closed too early could be detected at
the wrong place, it was possible to get a 502/SH instead of a silent close,
preventing the client to safely retries its request.

Thanks to this patch, we are able to silently close the client connection in
this case and eventually to perform a L7 retry.

This patch must be backported as far as 2.8.