RFR: 8316337: (bf) Concurrency issue in DirectByteBuffer.Deallocator [v4]

[Per Minborg]

> This PR proposes to make fields of implementations of the interface `DirectByteBuffer.Deallocator` `final` ensuring safe publication and immutability.

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Make unmappers truly immutable
 - Revert to anonymous UmapperProxy class

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15784/files
  - new: https://git.openjdk.org/jdk/pull/15784/files/64e94380..af043d02

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15784&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15784&range=02-03

  Stats: 51 lines in 3 files changed: 0 ins; 23 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/15784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15784/head:pull/15784

PR: https://git.openjdk.org/jdk/pull/15784