__init__(), close(): Remove self._references since we sniff the pickle
instead.

_decref() -> _zaprevision(): Also, changed implementation since we
observe that it can't really be shared with the _decref() for pack,
and we're moving to a non-destructive undo instead.  So make this
function (presumably) useful for pack() and cyclic garbage
collection.  We'll see...

undo(): Mark this TBR -- to be removed -- but don't (yet) remove it.