RFR: Implement heuristics to switch between merged and separate update-refs phase

[Aleksey Shipilev]

On 05/08/2017 05:12 PM, Roman Kennke wrote:
> This also means that, at least when running with adaptive heuristics
> (the default), the argument -XX:+/-ShenandoahUpdateRefsEarly is unused.

Um. I wonder if there is an option to turn off UR unconditionally. Should we
rewire that flag have three values: always-on, always-off, adaptive?

> http://cr.openjdk.java.net/~rkennke/merge-uprefs/webrev.01/

This seems redundant:
  672     double cycle_gap_max = _cycle_gap_history->maximum();

Otherwise good.

-Aleksey