in for nexus nodes (= ESI subrequests), we fini the pesi vdp (and,
conceptually, the buf also, but that requires details still) when we are
done with parsing, such that any other vdps are still in place.

Also we push a vdp which pushes bytes up one parent like ved_ved in
varnish-cache.

So we should now have a VDP chain for each subreq basically matching
that of varnish-cache