RFR: (sh/8): [backport] 8226757: Shenandoah: Make traversal and passive modes explicit

[Roman Kennke]

This backports the machinery that introduces ShenandoahGCMode, in
preparation for the Traversal GC backport. Contrary to the corresponding
11+ changes, this does not include the traversal mode. However, I will
push this change together with the big traversal GC backport.

Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8226757-jdk8/webrev.00/

Testing: hotspot_gc_shenandoah

Can I please get a review?

Roman