RFR: Fix number of threads used for initial evacuation

[Roman Kennke]

in ShenandoahHeap::evacuate_and_update_roots() we're using
_max_parallel_workers to initialize the roots processor (and thus the
StrongRootsScope), but we're not updating the workers to that (afaict).
This patch makes the root processor use the currently active number of
workers (should be the same as final-mark workers set earlier during
that pause). Ok?

http://cr.openjdk.java.net/~rkennke/init-evac-workers/webrev.00/

Roman