RFR (sh/jdk11): Backport remaining changes

[Roman Kennke]

This completes the recent round of backports to sh/jdk11. It includes
refactorings in roots processing and traversal testing improvements.

Changes:
http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-08-04/changes.txt
Full webrev:
http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-08-04/webrev.00/

Testing: hotspot_gc_shenandoah

Roman