RFR: 8297495: j.u.concurrent updates for JDK 20 [v2]

[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.

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 six additional commits since the last revision:

 - Expand test
 - Adjust wording to make it clear that the number of tasks is non-negative
 - Merge
 - Improve javadoc
 - Merge
 - Initial commit

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11319/files
  - new: https://git.openjdk.org/jdk/pull/11319/files/b1415b2e..a1ee40e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11319&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11319&range=00-01

  Stats: 14188 lines in 478 files changed: 9499 ins; 2646 del; 2043 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