RFR: Enhancement on traversal code root scanning

[Roman Kennke]

Looks good to me.

Roman

> 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