RFR: Upstreaming OWST task termination (Shenandoah changes)

[Roman Kennke]

I think the changes look good.

I'm wondering if we should push it to upstream first, then cherry-pick
the change ASAP and add the Shenandoah pieces then. This seems the least
disruptive path. Upstream may want stuff changed around, and from there
it gets messy...

Roman

> Hi,
> 
> Please review Shenandoah changes that will be affected by JDK-8204947.
> Also, please take a look of OWSTTaskTerminator, see if there are any
> faults.
> 
> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/upstream_owst/webrev.00/index.html
> 
> 
> Test:
>   tier3_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu