RFR: Fix recycled regions zapping

[Aleksey Shipilev]

Turns out, it is not enough to call ContiguousSpace::clear(true) to get zapping.
The default version calls into zap_unused, this zaps everything between top and
high watermark. And we do not set any watermark. So, as suggested in the docs,
we need to call mangle_unused_area_complete() after resetting top. This will
mangle everything between top() and end().

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/zapping-enable/webrev.01/

ZapUnusedHeapArea is true in debug, which makes tests run around 20% longer.

Testing: hotspot_gc_shenandoah, failing tests

Thanks,
-Aleksey