RFR: Upstreaming OWST task termination (Shenandoah changes)

[Zhengyu Gu]

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