RFR: Integrate upstream OWST task terminator to Shenandoah

[zgu at redhat.com]

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