RFR: 8302899: Executors.newSingleThreadExecutor can use Cleaner to shutdown executor [v2]

[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 incrementally with one additional commit since the last revision:

  Fix typo in comment, remove blank line

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12675/files
  - new: https://git.openjdk.org/jdk/pull/12675/files/1c771f26..3bb4d0cd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12675&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12675&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 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