RFR: 8365407: Race condition in MethodTrainingData::verify()

[Igor Veresov]

This change fixes multiple issue with training data verification. While the current state of things in the mainline will not cause  any issues (because of the absence of the call to `TD::verify()` during the shutdown) it does problems in the leyden repo. This change strengthens verification in the mainline (by adding the shutdown verify call), and fixes the problems that prevent  it from working reliably.

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Commit messages:
 - Fix TD verification

Changes: https://git.openjdk.org/jdk/pull/26866/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365407
  Stats: 84 lines in 6 files changed: 52 ins; 16 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/26866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26866/head:pull/26866

PR: https://git.openjdk.org/jdk/pull/26866