RFR: Verify marked objects

[Aleksey Shipilev]

Our previous bug hunt identified a white spot in Verifier.

Current Verifier only checks reachable objects, but there are objects that GC
scans even though they are not reachable, because the marking data may be
imprecise -- for example objects allocated past TAMS. This change adds the
additional pass over marked objects to Verifier:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-marked/webrev.01/

It also moves the "After Partial" verification after the region recycling,
because it would expectedly fail when scanning the "marked" objects in
non-recycled CSet otherwise.

Testing: hotspot_gc_shenandoah, specjbb

Thanks,
-Aleksey