RFR: 8244551: Shenandoah: Fix racy update of update_watermark

[Roman Kennke]

>> http://cr.openjdk.java.net/~rkennke/JDK-8244551/webrev.01/
> 
> Nice.
> 
> *) What's the point of this cast?
> 
>  133   *const_cast<HeapWord**>(&_update_watermark) =  w;

Make access non-volatile. You think it's too much?

> *) Nit: message can be shorter "Should be at Shenandoah safepoint"
> 
>  131   assert(SafepointSynchronize::is_at_safepoint(),
>  132          "only call set_update_watermark_at_safepoint at safepoint");

Ok, will change before push.

Roman