RFR: 8360884: Better scoped values

[Andrew Haley]

Scoped values cannot be used early in the JDK boot process because of some dependencies on System.getProperty(). This repen dency should be removed in a way that allows scoped values to be created (but not necessarily bound) at any stage during boot.

Also, Scoped Value's constructor has a synchronized block, which limits the use of scoped values at runtime. Constructing a scoped value should be a thread-local operation.

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Commit messages:
 - 8360884: Better scoped values
 - 8360884: Better scoped values
 - 8360884: Better scoped values

Changes: https://git.openjdk.org/jdk/pull/26019/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26019&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360884
  Stats: 97 lines in 3 files changed: 55 ins; 5 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/26019.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26019/head:pull/26019

PR: https://git.openjdk.org/jdk/pull/26019