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

[Aleksey Shipilev]

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)

-- 
Thanks,
-Aleksey