RFR: JDK-8302666: Replace CHM with VarHandle in ForeachOrderedTask [v2]

[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:

  Write the initial value of the next reference without using the VarHandle

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12320/files
  - new: https://git.openjdk.org/jdk/pull/12320/files/d07190ab..3b38f942

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12320&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12320&range=00-01

  Stats: 8 lines in 1 file changed: 4 ins; 3 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