RFR: JDK-8302666: Replace CHM with VarHandle in ForeachOrderedTask

[Viktor Klang]

I noticed when looking at the code that there was no real need to use a CHM to perform the tracking of activation in an ordered fashion on ForEachOrderedTask, but instead a VarHandle can be used, reducing allocations and indirection.

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Commit messages:
 - JDK-8302666: Replace CHM with VarHandle in ForeachOrderedTask

Changes: https://git.openjdk.org/jdk/pull/12320/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12320&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302666
  Stats: 25 lines in 1 file changed: 17 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12320.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12320/head:pull/12320

PR: https://git.openjdk.org/jdk/pull/12320