RFR: Verifier should use non-optimized root scans

[Aleksey Shipilev]

Dealing with bugs in the optimized code cache iterator is bad without having a
verifier that does *not* walk through the iterator in question. It makes sense
to use "slow" root scan to verify everything without touching
Shenandoah-specific code.

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

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey