Shenandoah: Used wrong closure to mark concurrent roots

During concurrent mark phase, there should not have forwarded objects in roots. Therefore, it should use ShenandoahMarkRefsClosure to mark concurrent roots, instead of ShenandoahMarkResolveRefsClosure.

Note: this is *not* a correctness bug, but performance one, as ShenandoahMarkResolveRefsClosure closure unnecessarily resolves forwarding pointers, where always resolved to themselves.