RFR (sh/11): [backport] 8229921: Shenandoah: Make Traversal mode non-experimental

[Roman Kennke]

I'd like to backport this quick so that it's in before I'm heading out
the next two weeks:

http://cr.openjdk.java.net/~rkennke/JDK-8229921-sh11/webrev.00/

Testing: hotspot_gc_shenandoah looking good

Ok?

Roman