MINOR: http-fetch: Add a sample to retrieve the server status code

MINOR: http-fetch: Add a sample to retrieve the server status code

The code returned by the "status" sample fetch is the one in the HTTP
response at the moment the sample is evaluated. It may be the status code in
the server response or the one of the HAProxy reply in case of error, deny,
redirect...

However, it could be handy to retrieve the status code returned by the
server, when a HTTP response was really received from it. It is the purpose
of the "server_status" sample fetch. The server status code itself is stored
in the HTTP txn.