[wip] Trace requests

I don't see why there would be security implications. It is an unpredictable value (not incrementing or sth. like that). It's not a hash of anything interesting (UUID). If a UUID is insecure to expose, we're screwed anyway.

Varnish (at least 2 and 3) also logs a request identifier: https://www.varnish-cache.org/docs/2.1/faq/http.html