RFR (S) 8241673: Shenandoah: refactor anti-false-sharing padding

[Roman Kennke]

Looks good.

Thanks,
Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241673
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241673/webrev.01/
> 
> Together with the cleanup, it drops the padding size to 64 bytes, thus optimizing footprint.
> sizeof(ShenandoahHeapRegion) drops from 264 to 200 bytes. Init/final mark pauses that deal with lots
> of regions are improving for about 50us each.
> 
> Testing: hotspot_gc_shenandoah; eyeballing gc-stats from stress runs; benchmarks (running)
>