RFR: 8341903: Implementation of Scoped Values (Fourth Preview)

[Andrew Haley]

The fourth preview of scoped values.

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Commit messages:
 - Scoped Values API changes
 - Scoped Values API changes
 - Scoped Values API changes

Changes: https://git.openjdk.org/jdk/pull/21456/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341903
  Stats: 159 lines in 5 files changed: 5 ins; 73 del; 81 mod
  Patch: https://git.openjdk.org/jdk/pull/21456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21456/head:pull/21456

PR: https://git.openjdk.org/jdk/pull/21456