RFR: Incremental-update option

[Roman Kennke]

The traversal mode uses incremental-update-style traversal. It might be
useful to pair that with our normal-mode operation (3-phase). The
following patch is a prototype of this, that I would like to give some
testing in shenandoah/jdk:

http://cr.openjdk.java.net/~rkennke/iu/webrev.00/

It adds a new mode called ShenandoahIUMode -XX:ShenandoahGCMode=iu and
wires the default ShenandoahGCMode to iu to give it maximum exposure in
testing pipelines.

Testing: hotspot_gc_shenandoah is all clean :-) Let's give it some more
runs on our CIs.

Ok to push to shenandoah/jdk ?

Roman