RFR (S) 8232573: Shenandoah: cleanup and add more logging for in-pause phases

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8232573

Fix:
  https://cr.openjdk.java.net/~shade/8232573/webrev.01

This improves profiling for pauses, fixing issues recently found when doing some performance
investigations and development work.

Testing: hotspot_gc_shenandoah, eyeballing -Xlog:gc+stats

-- 
Thanks,
-Aleksey