class, we save the class mosule and class name.  This allows us to
handle import errors ourself.

Added method to perform incremental gc.

Changed incremental collection effort algorithm to be based on
difference between actual and target size, rather than ration.

constructor when deactivating.

of all C Persistent objects.

Renamed _p___reinit__ to _p_deactivate.
Updated _p_changed attribute protocol. This will allow us to get rid
of _p_state and maybe someday __changed__().