Cherry-pick: concurrent evacuation related changes

[Zhengyu Gu]

Hi,

I would like to cherry-pick concurrent evacuation related changes to 
shenandoah/jdk, and continue concurrent class unloading work here, while 
we stabilize concurrent evacuation changes in jdk/jdk.

JDK-8225514: Shenandoah: ShenandoahCodeRoots should inherit from AllStatic

JDK-8225572: Shenandoah: Move JNIHandles root out of serial roots

JDK-8225550: Shenandoah: Prevent SH::object_iterate() call's side-effects

JDK-8225573: Shenandoah: Enhance ShenandoahVerifier to ensure roots 
to-space invariant

JDK-8225582: Shenandoah: Enable concurrent evacuation of JNIHandles

JDK-8226413: Shenandoah: Separate root scanner for SH::object_iterate()

JDK-8225590: Shenandoah: Refactor ShenandoahClassLoaderDataRoots API

JDK-8226586: Shenandoah: No need to pre-evacuate roots for degenerated GC

JDK-8226311: Shenandoah: Concurrent evacuation of OopStorage backed weak 
roots

JDK-8226310: Shenandoah: Concurrent evacuation of CLDG

JDK-8224151: Test 
vmTestbase/nsk/sysdict/vm/stress/chain/chain007/chain007.java might hang 
with release binaries

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/conc_evac/webrev.00/

Test:

   hotspot_gc_shenandoah (fastdebug and release)

Okay?

Thanks,

-Zhengyu