lb/ForwardHttpRequest: add reference to the request pool
lb/ForwardHttpRequest: add reference to the request pool

Fixes crash caused by commit 99077941be74afb8da24dfa240aa38ccbcd99823;
that commit is correct, but dereferencing HttpServerRequest is illegal
after the response was sent.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy