Strict compatibility with Varnish 6.2.0.

To reduce risks concerning portability and compatibility, and to
avoid accidentally depending on non-standard features.

- Use WS_Reserve() instead of WS_ReserveAll().
- Add HSH_Cancel() to the "foreign" code.

when we prune the tree, we may the current node containing the link
to the next unpending node may get freed, so we need to iterate using
the _SAFE variant which saves it.

the code is correct in varnish-cache, the VDP_bytes(req, VDP_NULL, ...) calls do not return.

This reverts commit ff7d20a2.

because the ESI request needs to stay alive until all nodes below have
been delivered, now that we got tree pruning, we can take them from the
ESI requests workspace.

See long comment in src/node.c for more detail and evidence collection
for the tuneables.

once a subtree is completely delivered, we do still need the T_NEXUS at
the head of the (sub)tree for integrity (linkage to its siblings), but
anything below can be completely fini'd and freed.

this is a bad leftover from when we still used ALLOC_JOB/FREE_OBJ for
nodes and anyway we must not just free a node which is still linked in
an otherwise alive tree

During code restructuring earlier in
77e9559d I overlooked that VTIM_real()
may not be declared where debug output is used

solve by moving the accumulation to the destructor

it is in the way of my torture testing and does not smell like something
we have caused..

this avoids a potential panic for an out of workspace condition.

Also, we move error handling to a place where it is much simpler