[11u] RFR (S) 8235324: Dying objects are published from users of CollectedHeap::object_iterate

[Aleksey Shipilev]

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8235324
   https://hg.openjdk.java.net/jdk/jdk/rev/35d8d9b65744

The patch applies almost cleanly to 11u. The only conflict I have to resolve is in zBarrier.hpp, 
where context is different. The patch itself is simple: keep-alive things that we touch with 
object_iterate, in case we load them with non-keepalive barriers.

11u variant:
   https://cr.openjdk.java.net/~shade/8235324/webrev.11u.01/

It requires two follow ups that unbreak Shenandoah, to be backported at the same time
  8237369: Shenandoah: failed vmTestbase/nsk/jvmti/AttachOnDemand/attach021/TestDescription.java test
  8237392: Shenandoah: Remove unreliable assertion

Testing: tier{1,2,3} with {G1, Shenandoah, ZGC, Parallel}; vmTestbase_nsk_jvmti with {G1, 
Shenandoah, ZGC, Parallel}

-- 
Thanks,
-Aleksey