It turns out that the cause of complication in backend request line
construction is a absolute-form in HTTP/1 request.  In HTTP/2, we have
separated pseudo-header fields and no problem at all.  In this commit,
we parse request URI in HTTP/1 frontend and extract values from it to
make backend logic simpler.  This patch removes host header field
emission in HTTP/2 backend if :authority is emitted.  It also rewrites
host header field with authority part in absolute-form URI as per RFC
7230.