RFR: 8284199: Implementation of Structured Concurrency (Incubator) [v4]

[Alan Bateman]

> This is the implementation of JEP 428: Structured Concurrency (Incubator).
> 
> This is a non-final API that provides a gentle on-ramp to structure a task as a family of concurrent subtasks, and to coordinate the subtasks as a unit.

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:

 - Fix typo in javadoc
 - Merge
 - Javadoc updates, add to jdk_loom test group
 - Add statement to close about thread termination
 - Use {@code ...}, replace task->subtask, fix typos, add jls ref
 - Merge
 - @ignore StructuredThreadDumpTest until test infra in place
 - Refresh
 - Merge
 - Initial commit

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8787/files
  - new: https://git.openjdk.java.net/jdk/pull/8787/files/4fc454a9..d11b24bc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8787&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8787&range=02-03

  Stats: 33428 lines in 507 files changed: 6180 ins; 25559 del; 1689 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8787.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8787/head:pull/8787

PR: https://git.openjdk.java.net/jdk/pull/8787