RFR 8228720: Shenandoah: Implementation of concurrent class unloading

[Zhengyu Gu]

Please review this implementation of Shenandoah concurrent class unloading.

This patch is built on top of concurrent nmethod evacuation and 
disarming in early patch. It moves class unloading and nmethod liveness 
check into concurrent evacuation phase.


Bug: https://bugs.openjdk.java.net/browse/JDK-8228720
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228720/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release) on Linux x86_64
   specjvm (fastdebug and release) on LInux x86_64.

Thanks,

-Zhengyu