RFR (8u-backport): Implement early update references phase

[Aleksey Shipilev]

On 05/09/2017 07:14 PM, Roman Kennke wrote:
> This is a straight backport of the early update references phase and all
> related code from JDK9.
> 
> http://cr.openjdk.java.net/~rkennke/backport-uprefs/webrev.00/
> <http://cr.openjdk.java.net/%7Erkennke/backport-uprefs/webrev.00/>
> 
> Testing: hotspot_gc_shenandoah, specjvm compiler
> 
> Ok?

Nice and tidy, thanks.

Comparing with 9, these are the interesting differences, which might be good to
fix in this iteration:

 *) ShenandoahHeuristics::record_gc_start/end() seem to be missing?
 *) ShenandoahHeap::update_refs_in_progress_addr() seems to be missing?

Thanks,
-Aleksey