RFR: Integrate upstream OWST task terminator to Shenandoah

[Roman Kennke]

The patch looks ok. But it's lacking the JDK part. Please push the whole
thing to Shenandoah's repo, otherwise it'll be temporarily broken. Merge
from upstream will sort it out again.

Thanks,
Roman


> Upstream "JDK-8214458: Port ShenandoahTaskTerminator to mainline and
> make it default" has been reviewed and is ready for pushing once CSR is
> approved.
> 
> Let's get a head start and review Shenandoah side of change based on
> latest JDK-8214458 patch.
> 
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/upstream_owst/webrev
> .01/
> 
> Test:
>   tier1, tier2 and tier3 gc_shenandoah
>   fastdebug and release
> 
> Thanks,
> 
> -Zhengyu
>