RFR (S) 8241668: Shenandoah: make ShenandoahHeapRegion not derive from ContiguousSpace

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241668

This is similar to what G1 did in the relevant cleanup (JDK-8189737). There is no reason to carry
the cruft from ContiguousSpace superclass, when we just need to pull down a few fields and auxiliary
methods from there.

sizeof(ShenandoahHeapRegion) went down from 328 to 264 bytes.

Webrev:
  https://cr.openjdk.java.net/~shade/8241668/webrev.01/

Testing: hotspot_gc_shenandoah, serviceability/sa with Shenandoah

-- 
Thanks,
-Aleksey