RFR (S) 8241668: Shenandoah: make ShenandoahHeapRegion not derive from ContiguousSpace

[Roman Kennke]

>> I don't think we need block_start() block_size() and block_is_obj()
>> (ShHeap and ShHeapRegion) for anything. Correct me if I'm wrong.
> 
> We do need it: hs_err and verifier printing enters here. It is not obvious with templates, but you
> can follow it from ShenandoahHeap::print_location.

Eh. Ok then. I thought those are relics of when we tried to do heap
iteration like serial GC (in ancient history).
Good!

Thanks,
Roman