RFR (S) 8242641: Shenandoah: clear live data and update TAMS optimistically

[Roman Kennke]

Looks good.

Thanks,
Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242641
> 
> After CM-with-UR removal, we are open to clear liveness data and update TAMS
> concurrently/optimistically. Shaves off a few microseconds on large number of regions.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8242641/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, eyeballing benchmarks
>