RFR: 8241081: Shenandoah: Do not modify update-watermark concurrently

[Roman Kennke]

>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8241081/webrev.02/
> 
> Looks good.
> 
> One tiny thing: can you please read _update_watermark into local field, assert that value and return
> the same one? Instead of doing effectively three memory accesses, which might disagree.

Ok like this?

http://cr.openjdk.java.net/~rkennke/JDK-8241081/webrev.03/