Repeating the test in e00019.vtc with HTTP/2 fails both here and
for standard Varnish. My understanding of HTTP/2 is too poor for
me to know whether there is a bug in a Varnish, or if I am not
applying HTTP/2 correctly. More details in the comments of the vtc.

Setting this aside for now, but we should come back, but we should
come back to it (and if necessary file a Varnish bug report).

because the push phase of unpending happens outside the tree lock, we
need to ensure only one thread is ever entering unpending

this is in preparation for future changes, right now, unpending is only
ever called from the topreq thread

This is the first step towards bringing back safe front pushes: For a
thread in vdp_push bytes, it is safe to push upwards if it owns all
ST_OPEN T_NEXUS nodes upwards or if they are ST_CLOSED or ST_DELIVERED.

more to come in later commits

it is only needed in the esi parser and for some sanity checks

much of the rest of the unpend design and state management assumes that
nexuses got nodes below.

By all means we do not want to start removing nodes from the tree,
because that would introduce even more sync or races with unpending, so
the logical solution is to turn the node into a zero T_DATA node.