RFR: 8279917: Refactor subclassAudits in Thread to use ClassValue

[Roman Kennke]

Thread.java would benefit from a refactoring similar to JDK-8278065 to use ClassValue instead of the somewhat problematic WeakClassKey mechanism.

Testing:
 - [x] tier1
 - [x] tier2
 - [x] tier3

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Commit messages:
 - 8279917: Refactor subclassAudits in Thread to use ClassValue

Changes: https://git.openjdk.java.net/jdk/pull/7054/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7054&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279917
  Stats: 84 lines in 1 file changed: 1 ins; 76 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7054.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7054/head:pull/7054

PR: https://git.openjdk.java.net/jdk/pull/7054