RFR: enqueue barrier + some other things

[Roland Westrelin]

> Could we _not_ move the code around when changing it? The 3.5 KLOC change in shenandoahSupport is
> extremely hard to review, and I think it could be better.

The code was moved because I tried to properly abstract it in its own
class and to move everything that was related together rather than have
pieces all over the place.

Moving the code around adds noise but, realistically, I don't think that
part of the change is reviewable anyway.

Roland.