RFR: 8302845: Replace finalizer usage in JNDI DNS provider with Cleaner

[Aleksei Efimov]

JNDI `DnsClient` has a finalize method to close its internal datagram channel selector.
 The change proposed here replaces it with a cleaner to close the selector once the `DnsClient`
  instance becomes phantom reachable. 

The change was tested with `jdk-tier1` to `jdk-tier3` test sets which showed no failures.

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Commit messages:
 - 8302845: Replace finalizer usage in JNDI DNS provider with Cleaner

Changes: https://git.openjdk.org/jdk/pull/13837/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13837&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302845
  Stats: 26 lines in 3 files changed: 12 ins; 10 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13837.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13837/head:pull/13837

PR: https://git.openjdk.org/jdk/pull/13837