RFR: avoid extra park of vthread when using fixed thread pool [v2]

[Miao Zheng]

On Mon, 30 Aug 2021 11:40:03 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Miao Zheng has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.
>
> The CustomScheduler test was indeed failing with this change but it's due to a test issue where it runs the scheduler tasks on the current thread. I've remove that case from the test so I think we are good to go now.

@AlanBateman Thanks for your review.

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PR: https://git.openjdk.java.net/loom/pull/59