RFR: Verify liveness data

[Aleksey Shipilev]

It shall make me sleep better knowing that our liveness calculation during concurrent mark is
verified. Breaking liveness calculation may have performance implications for heuristics (e.g. not
treating garbage-first regions well), and functional bugs (e.g. reclaiming immediate garbage
regions, while they are not really garbage).

Fix:
 http://cr.openjdk.java.net/~shade/shenandoah/verifier-liveness/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey