Shenandoah: Prevent recycle-assist during conc-roots

In ShFreeSet::allocate_humonguous() we try_recycle_trashed() without considering if conc-roots is in progress (like we do in try_allocate_in()). We must not recycle anything before conc-roots is done, because we need those unreachable objects during conc-reference-processing or conc-class-unloading. This may lead to crashes and is possibly extremely rare: it hasn't shown up anywhere, yet.