RFR: 8351000: StringBuilder getChar and putChar robustness

[Roger Riggs]

Improve StringBuilder getChar and putChar robustness with racy updates to StringBuilder. 
Test racing threads with append, insert, and reading characters.

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Commit messages:
 - 8351000: StringBuilder getChar and putChar robustness

Changes: https://git.openjdk.org/jdk/pull/24951/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24951&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351000
  Stats: 314 lines in 2 files changed: 309 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/24951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24951/head:pull/24951

PR: https://git.openjdk.org/jdk/pull/24951