The mark context is shared, but each generation maintains a separate flag (`ShenandoahGeneration::_is_marking_complete`) to indicate status of the mark. The remembered set scan will use the mark bitmap only if the old generation mark is complete. Otherwise, it walks the old regions and requires them to be parseable.

Presently, we make the heap parseable after global marking finishes, but this work could be deferred to the start of an old mark if we had global mark record completion of old marking.