RFR: Make update-refs cancellable

[Zhengyu Gu]

Looks good.

-Zhengyu

On 03/29/2017 11:02 AM, Roman Kennke wrote:
> This makes update-refs respect cancellation due to OOM, and
> short-circuit it to arrive at full-gc faster.
>
> It does *not* implement short-circuiting for the case where heuristics
> would want to start a new GC cycle. This is significantly more difficult.
>
> http://cr.openjdk.java.net/~rkennke/cancel-update-refs/webrev.00/
> <http://cr.openjdk.java.net/%7Erkennke/cancel-update-refs/webrev.00/>
>
> Roman
>
>