RFR [11u, 8u backport] Simplify Shenandoah task termination in aborted paths

[zgu at redhat.com]

Backport Shenandoah task termination logic from JDK12.

JDK11u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8215220/jdk11u/webre
v.00/


JDK8u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8215220/jdk8u/webrev.
00/


Test:
  tier1, 2, 3 gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu