RFR: Optimize heap verification

[Aleksey Shipilev]

Hi,

There are few easy things to improve in verification performance- and sanity-wise:

 *) Use non-synchronized Stack instead of SCM Queue. This helps performance, and
detaches verifier from relying on optimized Shenandoah implementation.

 *) Use single-oop task holder. This again helps performance, and keeps us away
from Shenandoah-optimized implementation, which deals with workstealing arrays
better. This does not apply to single-threaded verification.

 *) Peel verification method with test, to aid its inlineablity.

 *) Use BrooksPointer::get_raw to avoid excess asserts inside ::forwardee, which
we will assert in verification anyway.

All done here:
 http://cr.openjdk.java.net/~shade/shenandoah/verifier-optimize/webrev.01/

Testing: hotspot_gc_shenandoah

-Aleksey