RFR: 8279917: Refactor subclassAudits in Thread to use ClassValue [v2]

[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

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Remove obsolete comment

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7054/files
  - new: https://git.openjdk.java.net/jdk/pull/7054/files/0e495c4f..958e4e84

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7054&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7054&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 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