RFR: Verification levels

[Aleksey Shipilev]

Chasing the root set bug with Verifier and "aggressive" is gruesome, because we
scan lots of things except the root set. It makes sense to introduce
verification levels, so that one can select to check only the subset of
interesting things, e.g. the roots alone.

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-level/webrev.01/

Testing: hotspot_gc_shenandoah

-Aleksey