RFR: Fail early when critical barriers are disabled

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/check-barriers/webrev.01/

This are actually two changes.

*) Separate "passive" heuristics from "adaptive", by subclassing the root Heuristics, and
implementing cset selection for degenerated-gc path. This path is actually more efficient than the
one in adaptive, because it can capitalize on the available free space after immediate garbage
reclamation. The motivation to have the change in this bunch is to allow "passive" to check barriers
on its own.

*) Fail early when critical barriers are disabled, by letting heuristics check the configuration
before proceeding. This allows checking for normal-concurrent, traversal, and passive operation
modes. This makes diagnostic barrier flags much safer to use. New test validates the behavior.

Testing: hotspot_gc_shenandoah, adhoc passive benchmarks

Thanks,
-Aleksey