RFR: 8316641: VarHandle template classes can share code in the base class

[Chen Liang]

VarHandle implementations have many static fields and methods that can be pulled to the common superclass to avoid repeated initialization and code duplication.

In addition, the Unsafe-based Buffer field access are replaced by usage of public methods or JavaNioAccess.

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Commit messages:
 - Clean up VarHandle template classes

Changes: https://git.openjdk.org/jdk/pull/15854/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15854&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316641
  Stats: 136 lines in 5 files changed: 31 ins; 50 del; 55 mod
  Patch: https://git.openjdk.org/jdk/pull/15854.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15854/head:pull/15854

PR: https://git.openjdk.org/jdk/pull/15854