RFR: 8362123: ClassLoader Leak via Executors.newSingleThreadExecutor(...)

[Chris Dennis]

Executors shutdown via `shutdownNow()` should have their cleanables cleaned to prevent a classloader leak. This can happen if a classloader exists that both references the wrapped executor and is referenced by the delegate executor.

To quote @Martin-Buchholz:
> BTW: I find Cleaners much harder to use than old finalize, and it looks like I'm not the only one!

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Commit messages:
 - 8362123: ClassLoader Leak via Executors.newSingleThreadExecutor(...)

Changes: https://git.openjdk.org/jdk/pull/26296/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362123
  Stats: 54 lines in 2 files changed: 52 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26296.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26296/head:pull/26296

PR: https://git.openjdk.org/jdk/pull/26296