loosly on the experimental persistent class code from Zope 3.

No-longer use setklassstate.  Instead, just call _p_invalidate, as
with any other object.  This changes didn't break any tests, so I
assume that this was untested. :(

Changed the strategy for invalidating objects.  Non-ghostifiable
objects will load their state when they are invalidated.  We have to
worry about other invalidations that come in while this is happening.
See the comment in _flush_invalidations.

We need to force all invalidations to take this into account, by going
through _flush_invalidations.  I haven't done this yet, but I have
left some XXX comments in places where it needs to be done to remind
myself that this needs to be done.

No-longer use setklassstate.  Instead, just call _p_invalidate, as
with any other object.  This changes didn't break any tests, so I
assume that this was untested. :(

Change invalidation to not swallow errors. (Swallowing errors here was a
travesty!)

original state.

- browser for stored values for all UnIndex derived indexes

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- refactored batch size

the extra argument. Its parameter 'indexed_attrs'
can be string with comma delimited attribute names
or list/tuple

This fixes: # 1688

- len(self._unindex) as used in numObjects() might take too long

Import of the Zope package is now deprecated, but will
be supported until Zope 2.11.

Changed all imports of and references to the Zope to use Zope2 instead.