[11u] RFR (S) 8255457: Shenandoah: cleanup ShenandoahMarkTask

[Aleksey Shipilev]

Original RFE:
   https://bugs.openjdk.java.net/browse/JDK-8255457
   https://git.openjdk.java.net/jdk/commit/1215b1a8

This patch does not apply cleanly to 11u, because of the contextual differences. Most significant 
difference is that in HEAD JDK the affected class is trivially copyable, and the shared code already 
adjusted to accept that. 11u, however, would fail to compile due to odd cv-stripping casts. 
Therefore, I had to keep copying constructors and assignment operators.

11u webrev:
   https://cr.openjdk.java.net/~shade/8255457/webrev.11u.01/

Testing: Linux {x86_64, x86_32} hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey