RFR 8226413: Shenandoah: Separate root scanner for SH::object_iterate()

[Zhengyu Gu]

The patch is a prerequisite of concurrent CLD evacuation (JDK-8226310).
By moving root scanner for SH::object_iterate() out of generic root 
scanner, we can enforce stricter restriction on this specialized 
scanner. In particular, it should not clear CLDG's claim mask and should 
use _claim_none to iterate CLDG to avoid interfering concurrent CLDG 
iteration.


Bug: https://bugs.openjdk.java.net/browse/JDK-8226413
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8226413/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu