RFR: 8367857: Implement JEP 525: Structured Concurrency (Sixth Preview)

[Alan Bateman]

Updates for JEP 525.

- The join method is changed to invoke `Joiner::onTimeout` if a timeout is configured and the timeout expires before or while waiting.
- `Joiner.allUntil(Predicate`) is changed to allow join return the stream of all forked subtasks when the timeout expires.
- The `join` method is now specified to allow it be called again if interrupted.
- The configFunction parameter to the 3-arg open is changed from `Function<Configuration, Configuration>` to `UnaryOperator<Configuration>`.
- Joiner is no longer a `@FunctionalInterface`.

Most of the changes are to API docs and tests. Some links are changed to use double hash mark.

-------------

Commit messages:
 - Improve docs and review feedback
 - Merge branch 'master' into JDK-8367857
 - Add JEP number
 - Merge branch 'master' into JDK-8367857
 - Update
 - Merge branch 'master' into JDK-8367857
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/27392/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27392&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367857
  Stats: 493 lines in 5 files changed: 337 ins; 36 del; 120 mod
  Patch: https://git.openjdk.org/jdk/pull/27392.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27392/head:pull/27392

PR: https://git.openjdk.org/jdk/pull/27392