Log X-Analytics header of response instead of request

As varnish may mangle the X-Analytics header through backend (“mf-m”
key), or directly through vcl files (“zero” key), we're more
interested in the response's X-Analytics header than the request's
vanilla header.

modules/varnish/files/varnishncsa.default also uses the X-Analytics
header of the response in LOG_FORMAT.

Change-Id: If5bbc6f38b0674653bfbfe150b6ed8f30a98de15