The atomic counters caused a serious performance regression for failed promotions.
This change also combines the plab fields from `ShenandoahThreadLocalData` with the associated plab methods from `ShenandoahGenerationalHeap` into a more cohesive `ShenandoahPLAB` class.
This change also combines the plab fields from `ShenandoahThreadLocalData` with the associated plab methods from `ShenandoahGenerationalHeap` into a more cohesive `ShenandoahPLAB` class.