RFR (S) 8225046: Shenandoah metrics logs refactoring

[Aleksey Shipilev]

RFE:
 https://bugs.openjdk.java.net/browse/JDK-8225046

Shenandoah uses internal metrics to gauge progress in degenerated and full GC. The logging there can
be refactored a bit: it should clearly show the progress decisions made. Also, there is a bug in
displaying the external fragmentation.

Fix:
  http://cr.openjdk.java.net/~shade/8225046/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey