RFR (S): Refactor BrooksPointer utility class

[Aleksey Shipilev]

Hi,

When I was doing the fwdptr experiments, new asserts in BrooksPointer
helped to diagnose bugs. Also, the API is unfortunate in the way it
provides levity for doing fwdptr transitions we don't want. This is the
cleanup:
 http://cr.openjdk.java.net/~shade/shenandoah/brooks-asserts/webrev.01/

Testing: {fastdebug,release} hotspot_gc_shenandoah, jcstress
tests-custom -m quick (in progress)

Thanks,
-Aleksey