RFR: 8284199: Implementation of Structured Concurrency (Incubator)

[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.

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Commit messages:
 - @ignore StructuredThreadDumpTest until test infra in place
 - Refresh
 - Merge
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/8787/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8787&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284199
  Stats: 2801 lines in 10 files changed: 2800 ins; 0 del; 1 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