RFR: 8367857: Implement JEP 525: Structured Concurrency (Sixth Preview) [v2]

[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.

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 10 additional commits since the last revision:

 - Sync up from loom repo
 - Merge branch 'master' into JDK-8367857
 - Merge branch 'master' into JDK-8367857
 - 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:
  - all: https://git.openjdk.org/jdk/pull/27392/files
  - new: https://git.openjdk.org/jdk/pull/27392/files/66aca21e..c487fd60

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27392&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27392&range=00-01

  Stats: 23912 lines in 785 files changed: 12848 ins; 6356 del; 4708 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