RFR: Degenerated GC: rename enum, report degen reasons in stats

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/degenerated-logs/webrev.01/

The major improvement is classifying Degenerated GC reasons in -Xlog:gc+stats. Also renamed the enum
to be shorter.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey