RFR: Degenerating update-references phase

[Roman Kennke]

This patch implements degenerating of update-refs phase in much the same
way (and the same reasons) as we did for concurrent marking: when we run
OOM during (concurrent) update-references, it seems likely that we're
close to finish, and thus, instead of starting from scratch and doing a
full-gc, finish updating refs under the final-update-refs-pause, and
reclaim memory. When we do this, we're pessimizing adaptive heuristics,
so hopefully next time we have more room and not run into this.

http://cr.openjdk.java.net/~rkennke/degen-uprefs/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/degen-uprefs/webrev.00/>


Testing: hotspot_gc_shenandoah, specjvm compiler

Ok?

Roman