RFR: 8302899: Executors.newSingleThreadExecutor can use Cleaner to shutdown executor [v4]

[Alan Bateman]

> Executors.newSingleThreadExecutor returns a delegating ExecutorService that has finalizer to shutdown the underlying TPE when the wrapper is finalizable. It goes back to JDK 6 and JDK-6399443. This is the last non-empty finalizer in java.base. Removing it will likely lead to bug reports/complaints as the current behavior goes back to 2006. So the proposal is to just replace it with a Cleaner, trivially done in this case. As part of the changes, I've replaced the existing test with a more modern test that exercises more scenarios.

Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge
 - Improve SM scenario
 - Keep reference to Cleanable
 - Merge
 - Fix typo in comment, remove blank line
 - Replace older test
 - Initial commit

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12675/files
  - new: https://git.openjdk.org/jdk/pull/12675/files/3b135f09..449f72b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12675&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12675&range=02-03

  Stats: 1834 lines in 125 files changed: 1323 ins; 293 del; 218 mod
  Patch: https://git.openjdk.org/jdk/pull/12675.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12675/head:pull/12675

PR: https://git.openjdk.org/jdk/pull/12675