In Genshen impl, there are 4 marking completeness flags:
```
ShenandoahMarkingContext::_is_complete
ShenandoahGlobalGeneration::_is_marking_complete
ShenandoahYoungGeneration::_is_marking_complete
ShenandoahOldGeneration::_is_marking_complete
```

The ShenandoahMarkingContext::_is_complete should be in sync with ShenandoahGlobalGeneration::_is_marking_complete, but current code seems broken, ShenandoahMarkingContext::_is_complete is set to true in ShenandoahHeap::initialize but never reset in any other places.