RFR: 8297495: j.u.concurrent updates for JDK 20

[Alan Bateman]

The proposed updates for JDK 20 are:

- ForkJoinPool.externalSubmit 
- ForkJoinWorkerThread.getQueuedTaskCount 

These methods will be used to improve the Thread.yield implementation for virtual threads. The range of alternatives explored include not exposing an API and protected methods such as "offerSubmission". The class description speaks of "external clients" and "submissions from non-ForkJoinTask clients", hence the proposed naming and javadoc text.

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Commit messages:
 - Improve javadoc
 - Merge
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/11319/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11319&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297495
  Stats: 237 lines in 4 files changed: 233 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11319.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11319/head:pull/11319

PR: https://git.openjdk.org/jdk/pull/11319