RFR: 8283470: Update java.lang.invoke.VarHandle to use sealed classes

[Mandy Chung]

This patch changes VarHandle and its implementation hierarchy to use sealed classes.  All VarHandle permitted classes are package-private and either final or sealed abstract classes.

Please also review the CSR: https://bugs.openjdk.java.net/browse/JDK-8283540

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Commit messages:
 - 8283470: Update java.lang.invoke.VarHandle to use sealed classes

Changes: https://git.openjdk.java.net/jdk/pull/7926/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7926&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283470
  Stats: 58 lines in 5 files changed: 42 ins; 5 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7926.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7926/head:pull/7926

PR: https://git.openjdk.java.net/jdk/pull/7926