RFR: Implement protocol for safe OOM during evacuation handling

[Roman Kennke]

This implements a protocol for safe OOM during evacuation handling.

It's been explained and discussed here:
http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-February/005185.html

.. and in code.

Passes hotspot_gc_shenandoah and manual testing with
specjvm/-XX:+ShenandoahOOMDuringEvacALot modulo existing bug in
full-gc with pinned-region

Ok?

http://cr.openjdk.java.net/~rkennke/safe-oom-during-evac/webrev.07/

Roman