RFR: 8234768: Shenandoah: Streamline enqueueing runtime barriers

[Roman Kennke]

Shenandoah's runtime barriers for SATB pre-date the GC interface, and
have been very coarsly fitted into the new GC interfaces. It leaves room
for improvements:
- Rename methods to make more sense
- Group enqueueing barrier methods together
- Make them inlinable, including the ultimate enqueue() method
- Avoid barriers on DEST_UNINITIALIZED pre-barriers
- Benefit from static resolution of decorators when possible, and don't
generate barriers at all in those cases

As a bonus, add SATB and traversal store-value barriers to native oop
stores. This is not doing anything now, but will enable concurrent roots
scanning in the near future.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8234768
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8234768/webrev.00/

Can I please get a review?

Thanks,
Roman