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

[Roman Kennke]

Good, that seems useful. Patch looks good.

Roman

> 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
>