works by writing a metadata record which contains a special lrevid
(pickle pointer).  Upon loading a zombified object, naive applications
will get a KeyError as would be expected for loading a non-existant
object.

The actual exception though is ObjectDoesNotExist -- derived from
KeyError -- which contains a `revid' attribute pointing to the
revision of the object which zombified it.  Thus, by undoing this
revid, the object can be reborn to its initial state.

Specifically,

ObjectDoesNotExist: New exception, derived from KeyError.

load(): If an object's metadata has an lrevid with the special `dne'
value (64-bits full of 1's), the object is a zombie and
ObjectDoesNotExist is raised.

transactionalUndo(): If prevrevid for the revision we're undoing is
zero, it means that we're undoing the object's creation.  Instead of
raising an UndoError, write a new metadata record containing the
original record, but with lrevid == dne, and prevrevid == tid.

This change also contains initial (untested) support for pack(),
specifically,

_zaprevision(): pack doesn't happen in a transaction, so remove the
txn argument.  Add the referencesf argument, passed from the pack()
call.

pack(): Initial implementation.  Seems fairly simple really (too
simple? ;)