RFR(S) 8224210: Shenandoah: Refactor ShenandoahRootScanner to support scanning CSet codecache roots

[Zhengyu Gu]

Split the refactoring from early JDK-8224179 patch, as suggested by Aleksey.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224210
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224210/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   Failed CTW tests in traversal mode as expected.

Thanks,

-Zhengyu