RFR: Make update-refs cancellable

[Roman Kennke]

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