BUG/MEDIUM: http-ana: Deliver 502 on keep-alive for fressh server connection

BUG/MEDIUM: http-ana: Deliver 502 on keep-alive for fressh server connection

In HTTP keep-alive, if we face a connection error to the server while
sending the request, the error should not be reported, and the client-side
connection should simply be closed, so that client knows it can retry.

If the error happens during the connection stage, there is two cases. We
have a connection timeout or an allocation error. In this case, the 503
response must be skipped if it is not the first request on the client-side
connection. Or we have a connection error. In this case, the 503 response
must be skipped if it is a reused server connection. Otherwise, during the
connection stage, the 503-Service-unavailable response is delivered to the
client. The part works properly.

If the error happens after this stage, the 502-Bad-gateway response
delivering should only be based on the server-side connection status. For a
reused server connection, the client-side connection must be closed with no
reponses. However, for a fresh server-side connection, a 502-Bad-gateway
response must be delivered to the client. Unfortunately, This part is
buggy. Only the client-side connection state is considered and the response
is skipped if it is not the first request for the same client connection.

The bug is not so visbile in HTTP/1.1 but in H2 and H3 it is pretty annoying
because for a connection, requests are multiplexed, in parallels. It means
there is no first request. So, because of this bug, for H2 and H3,
502-Bad-gateway responses because of a connection error before receiveing
the response are always skipped.

To fix the issue, in http_wait_for_response() analyser, we must only rely on
SF_SRV_REUSED stream flag to skip the 502 response or not. This flag is set
if the server connection was reused.

The bug is their since a while. SF_SRV_REUSED flag was added in the version
1.5 especially to fix this kind of bug. But only the 503 case was fixed.

This patch should fix the issue #2285. It must be backported to every stable
versions.