MEDIUM: filters: Replace filter_http_headers callback by an analyzer

MEDIUM: filters: Replace filter_http_headers callback by an analyzer

This new analyzer will be called for each HTTP request/response, before the
parsing of the body. It is identified by AN_FLT_HTTP_HDRS.

Special care was taken about the following condition :

  * the frontend is a TCP proxy
  * filters are defined in the frontend section
  * the selected backend is a HTTP proxy

So, this patch explicitly add AN_FLT_HTTP_HDRS analyzer on the request and the
response channels when the backend is a HTTP proxy and when there are filters
attatched on the stream.
This patch simplifies http_request_forward_body and http_response_forward_body
functions.