LivePersistent can go to ghost state, because invalidation cannot be
ignored, i.e. they indicate the object has been changed externally.

This does not break our logic for ZBigFile and ZBigArray as
invalidations can happen only at transaction boundary, so during the
course of transaction those classes are guaranteed to stay uptodate and
thus not loose ._v_file and ._v_fileh (which is the reason they inherit
from LivePersistent).

it is ok to loose ._v_file and ._v_fileh at transaction boundary and
become ghost - those objects will be recreated upon going back uptodate
and will stay alive again during the whole transaction window.

We care only not to loose e.g. ._v_fileh inside transaction, because
loosing that data manager and thus data it manages inside transaction
can break synchronization logic and forget changed-through-mmap data.