RFR: 8278065: Refactor subclassAudits to use ClassValue [v4]

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

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Merge remote-tracking branch 'origin/JDK-8278065' into JDK-8278065
 - Avoid assert on unboxing Boolean

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6637/files
  - new: https://git.openjdk.java.net/jdk/pull/6637/files/6b37d520..78bda4b8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6637&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6637&range=02-03

  Stats: 4 lines in 2 files changed: 0 ins; 2 del; 2 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