What to expect (and test) with Gramps 6.0 beta

@kku shared a patch for the Optimizer class that resolves issues with several Rules and multi-stage Custom Filters. However, he is unsure WHY it helps. So the root cause needs more isolation.

Note that his Optimizer patch also makes the Home Person rule (_isdefaultperson.py) work again.
See 0013733: the “Active Person” filter rule crashes