Add function to retrieve worker_id from any context

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 the worker_id is not easily accessible in that context (the discover path).