RFR: Refactor and improve ShenandoahCodeRoots strategies

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/codecache-styles-rework/webrev.01/

This refactors our support for different code root styles, and improves style-2 a lot. Style-0 and
style-1 are just general cleanups. We would try and switch to a more optimal style-2 later, as
performance runs complete. But I would appreciate reviews for the code changes themselves meanwhile.

Testing: hotspot_gc_shenandoah with ShCodeRootsStyle={0,1,2}, benchmarks

Thanks,
-Aleksey