RFR: 8277394: Remove the use of safepoint_workers in reference processor

[Thomas Schatzl]

Hi all,

  please review this alternate implementation of 8277394. During the review of the first implementation (https://github.com/openjdk/jdk/pull/6453) the reviews did not like that the `WorkerThreads` were stashed away in the `ReferenceProcessor`.

This is an alternate implementation that implements the suggestion in the review comments, passing them along through the callers. While this requires a bit more changes, imo it better separates reference processor logic from the execution resources.

Testing: tier1-5

Thanks,
  Thomas

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Commit messages:
 - 8277394

Changes: https://git.openjdk.org/jdk/pull/25995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8277394
  Stats: 63 lines in 9 files changed: 18 ins; 26 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/25995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25995/head:pull/25995

PR: https://git.openjdk.org/jdk/pull/25995