RFR: Rework ClassUnloading* flags handling

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/class-unload-rework/webrev.01/

We have lost some of this during upstream merges, and we have not noticed because no tests verified
how we handle ClassUnloading* flags. This patch makes sure ClassUnloading* flags are treated well by
Shenandoah in every mode. New tests capture the intended behavior, including OOME when class
unloading is disabled and classes leak.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey