RFR: 8278351: Add function to retrieve worker_id from any context [v4]

[Per Liden]

On Mon, 13 Dec 2021 16:13:43 GMT, Per Liden <pliden at openjdk.org> wrote:

>> The `worker_id` argument passed to `WorkerTask::work(uint worker_id)` is sometimes needed in contexts where it today can't be easily accessed, mainly because it's not feasible to pass this argument through many layers of semi-unrelated code. To solve this, I propose that we make the current `worker_id` accessible though a static function `WorkerThread::worker_id()`.
>> 
>> By doing this, we also pave the way for ZGC to remove its `ZTask` and `ZThread`, which are helper class to make the `worker_id` easily accessible in any context.
>> 
>> At the same time I propose that we remove `WorkerThread::_id`. This id identifies the actual worker thread. There is currently only one user of `WorkerThread::_id`, the `ReferenceProcessor`. However, we should use `WorkerThread::worker_id()` here instead. The only reason it doesn't use `worker_id` today is that it's not easily accessible in that context (the discover path).
>> 
>> Testing: Passed Tier1-3
>
> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix Shenandoah build

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/6747