RFR: Make degenerated update-refs use region-set cursor to hand over work

[Roman Kennke]

Am 14.12.2017 um 19:06 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/ur-degen-cursor/webrev.01/
> 
> This is based on previous RFR that cleans up operations. For Degenerate GC to work, we want to drop
> cancellation flag right away, and do init-update-refs, followed by final-update-refs to finish the
> update refs work. But, final-update-refs would not finish work when cancellation is cleared.
> 
> Since work handover is tracked by regions cursor anyway, why don't we use that to signal available
> work? This also handles the case where cancellation is called when all threads have processed all
> regions during conc-update-refs, and reacted on cancellation at the end of the phase. Current code
> would make a futile attempt to whip up workers during final-update-refs, when we know there is no
> work left.

Ok