Follow-up to #3125 :

That PR added VDP_END, but always issued another VDP_bytes() call with
no data.

We now make it the responsibility of the object iterator to send
VDP_END and opportunistically try to issue it with the last bytes in
order to avoid the additional VDP_bytes() call with no data.

As noted by Martin in
https://github.com/varnishcache/varnish-cache/pull/3125#issuecomment-600620905
the advantage is that "in H/2 there is the need to send a frame with a
last bit set to mark the end of the stream" and in H/1 we could generate
the end chunk right after the last bit of data.

This change does not implement these optimizations yet, but prepares the
grounds.