RFR: 8260621: (jrtfs) ThreadLocal memory leak in ImageBufferCache when using jrtfs

[Athijegannathan Sundararajan]

Instead of BufferReference class, Map.Entry is used as pair implementation.
This avoids the metaspace leak seen via thread local.

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Commit messages:
 - added comment. generics cleanup.
 - 8260621: (jrtfs) ThreadLocal memory leak in ImageBufferCache when using jrtfs

Changes: https://git.openjdk.java.net/jdk/pull/3849/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3849&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260621
  Stats: 57 lines in 1 file changed: 30 ins; 7 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3849.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3849/head:pull/3849

PR: https://git.openjdk.java.net/jdk/pull/3849