RFR: JDK-8302666: Replace CHM with VarHandle in ForeachOrderedTask [v6]

[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.

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Removing unnecessary cast of argument to VarHandle getAndSet

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12320/files
  - new: https://git.openjdk.org/jdk/pull/12320/files/a5c6b0d0..2c8f0ab4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12320&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12320&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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