RFR(L) 8228665: Shenandoah: Implementation of Concurrent nmethod evacuation

[Zhengyu Gu]

This patch moves nmethod evacuation and disarming into concurrent phase.
However, nmethod's liveness is still checked at a pause.

This is the last step before we complete concurrent class unloading, 
where we move class loader and nmethod's liveness check into concurrent 
evacuation phase.

Given the scope of this changes, let's bake it in shenandoah/jdk first, 
before upstream to jdk/jdk.

Bug: https://bugs.openjdk.java.net/browse/JDK-8228665
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228665/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   specjvm (fastdebug and release)

Thanks,

-Zhengyu