nghttpx: Better handling EOF from downstream connection to tunnel stream
nghttpx: Add configurable access logging format

This commit adds functionality to customize access logging format in
nghttpx.  The format variables are inspired by nginx.  The default
format is combined format.
nghttpx: Fix heap-after-free crash in https upstream

Add Upstream::on_handler_delete() hook to safely write log for
HttpsUpstream.
nghttp: Fix compile error without libjansson