RFR: Enhancement on traversal code root scanning

[Zhengyu Gu]

Based on offline discussions and rkennke's early comments [1].

- Uses ShenandoahCsetCodeRootsIterator to scan code roots in init-traversal

- Removes code cache scanning in final-traversal.


Note:
- SPECjbb runs showed no noticeable changes in init-traversal and slight
   decreasing of final-traversal time.
   Results were indifference.

- Experimented seeding traversal with only evacuation roots, it showed
   no benefits (SPECjbb results tanked)


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

Test:

   tier3_gc_shenandoah (fastdebug and release)
   SPECjbb (release)


[1] 
http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-August/007141.html


Thanks,

-Zhengyu