RFR(S) 8224978: Shenandoah: Allows root verifier to verify some roots outside safepoints with proper locks

[Zhengyu Gu]

Please review this patch that allows root verifier to verify CLDG and 
code roots outside safepoints, with proper locks.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224978
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224978/webrev.00/

Test:
   hotspot_gc_shenandoah + ShenandoahVerify (fastdebug and release)

Thanks,

-Zhengyu