RFR: 8272985: Reference discovery is confused about atomicity and degree of parallelism

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes some (apparent) confusion between atomicity (of the discovery in the `ReferenceProcessor` sense) vs. the currently selected degree of parallelism?

For all four different combinations of atomicity and parallelism the `discovered` link in the `java.lang.ref.Reference` needs to be updated differently using a different kind of access, instead of just two based on atomicity.

I'll fix the use of `atomic` in `ReferenceProcessor` in a separate CR to hopefully remove the confusion for the reader too.

Testing: tier1-5, internal perf benchmarks without regressions

Thanks,
  Thomas

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Commit messages:
 - Some refactoring
 - Confused with atomic vs. MT
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5314/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5314&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272985
  Stats: 82 lines in 3 files changed: 44 ins; 20 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5314.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5314/head:pull/5314

PR: https://git.openjdk.java.net/jdk/pull/5314