RFR: 8278065: Refactor subclassAudits to use ClassValue

[Roman Kennke]

As a follow-up to #6375, this change refactors java.io.ObjectInputStream.Caches#subclassAudits and java.io.ObjectOutputStream.Caches#subclassAudits to use ClassValue instead of SoftReference, similar to what we did in #6375 for java.io.ObjectStreamClass.Caches#localDescs. Then we can now also remove the common machinery java.io.ObjectStreamClass#processQueue and java.io.ObjectStreamClass.WeakClassKey.

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

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Depends on: https://git.openjdk.java.net/jdk/pull/6375

Commit messages:
 - 8278065: Refactor subclassAudits to use ClassValue

Changes: https://git.openjdk.java.net/jdk/pull/6637/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6637&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278065
  Stats: 105 lines in 3 files changed: 2 ins; 87 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6637.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6637/head:pull/6637

PR: https://git.openjdk.java.net/jdk/pull/6637