RFR: 8372564: Convert StringDedup to use Atomic<T>

[Kim Barrett]

Please review this change to StringDedup to use Atomic rather than directly
applying AtomicAccess to volatile members.

Testing: mach5 tier1-5

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Commit messages:
 - stringdedup uses Atomic

Changes: https://git.openjdk.org/jdk/pull/28524/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28524&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372564
  Stats: 23 lines in 4 files changed: 4 ins; 3 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/28524.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28524/head:pull/28524

PR: https://git.openjdk.org/jdk/pull/28524