RFR: Fix ShenandoahHeapRegion initialization

[Roman Kennke]

Aleksey found out that ShenandoahHeapRegion::_critical_pins is never
initialized. I had a look and there are also some other fields in
ShenandoahHeapRegion uninitialized... we seem to be lucky that the OS
zeroes memory for us, and no other allocations interfered with us.
Don't like to rely on luck though, so here's the fix:

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

It does all initialization in the ShenandoahHeapRegion's initializer
list, and eliminates ShenandoahHeapRegion::initialize_heap_region().

Ok to push?

Roman