RFR (S) 8244226: Shenandoah: per-cycle statistics contain worker data from previous cycles

[Roman Kennke]

Looks good, thanks!

Roman

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8244226
> 
> See how it fails in the bug.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8244226/webrev.01/
> 
> Testing: hotspot_gc_shenandoah {fastdebug,release}, eyeballing gc logs
>