Add a request correlation ID to $req

This MR attempts to help make logs more useable by providing a request_id field in $req.

This request ID is set by mod_unique_id (Apache), or by a new configuration directive in Nginx. It allows to correlate access logs and error logs.

This new feature is not enabled by default, but easy to configured by changing the web server log format + enabling and configuring log4perl. It will also be used in #2941 (closed)