RFR: Make sure different Verifier levels work

[Aleksey Shipilev]

Setting -XX:ShenandoahVerifyLevel=0 currently fails with thread parity errors. This happens because
we always instantiate ShenandoahRootsProcessor, and on some paths we don't claim threads, leading to
this error.

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

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey