- require (Py)GetState and make it clear that it is called by
  persistency machinery only on non-ghost objects.
- make it clear that (Py)SetState is called by persistency machinery
  only on ghost objects.
- make it clear that DropState is called by persistency machinery only
  on non-ghost objects.

For btree PyGetState is marked as TODO which we'll fill incrementally
(the code is draftly ready, but there is no test for now).