RFR: Heap/matrix verification for all reachable objects

[Aleksey Shipilev]

Hi,

This is the verification code that works by asserting invariants on all
_reachable_ objects. This allows to verify both heap and the matrix without
having the complete prior mark. Which helps, for example, to assert the matrix
validity in partial collections.

See:
  http://cr.openjdk.java.net/~shade/shenandoah/matrix-verify-reachable/webrev.01/

Testing: hotspot_gc_shenandoah (fastdebug/release), selected performance workloads

Thanks,
-Aleksey