RFR: 8280041: Retry loop issues in java.io.ClassCache

[Aleksey Shipilev]

JDK-8277072 introduced java.io.ClassCache, but there seem to be at least two issues with it:
  - The cache cannot disambiguate between cleared SoftReference and the accidental passing of `null` value; in that case, the retry loop would spin indefinitely;
  - If retry loop would spin several times, every time discovering a cleared SoftReference, it would create and register new SoftReference on the ReferenceQueue. However, it would not *drain* the RQ in the loop; in that case, we might have the unbounded garbage accumulating in RQ; 

Attention @rkennke, @plevart.

Additional testing:
  - [x] Linux x86_64 fastdebug `java/io/ObjectStreamClass`
  - [x] Linux x86_64 fastdebug `tier1`
  - [ ] Linux x86_64 fastdebug `tier2`
  - [ ] Linux x86_64 fastdebug `tier3`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7092/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7092&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280041
  Stats: 6 lines in 1 file changed: 4 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7092.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7092/head:pull/7092

PR: https://git.openjdk.java.net/jdk/pull/7092