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

[Per Liden]

> 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

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6747/files
  - new: https://git.openjdk.java.net/jdk/pull/6747/files/67d3299c..d168e3c2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6747&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6747&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6747.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6747/head:pull/6747

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