ExtensionClass deletion protocol.

Added DECREF for object id, which is now an object.

destination version to fail to detect the new data.

invalidated objects.

Waaa.  I'll probably need to add a separate lock file.

methods did setattrs.

tries to minimize the size of the cache.

"volatile" attributes (starting with _v_) to cause database writes.

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