these optimizations are possible if Varnish-Cache accepts the
transport_hsh_cancel PR:

If the transport gets to decide when to call HSH_Cancel(), we can keep
references also to private leaf objects.

For ESI objects, which we also need to inspect when building the
delivery tree, we still need to make copies in pesi_buf_bytes because,
if the final flag to ObjIterate() is set, the storage engine can (and
usually will) free segments "behind" delivery.