RFR: 8302845: Replace finalizer usage in JNDI DNS provider with Cleaner [v4]

[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.

Aleksei Efimov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into JDK-8302845_DNS_client_replace_finalizer
 - Use cleanable to close the selector in DnsClient::close
 - fix comment
 - 8302845: Replace finalizer usage in JNDI DNS provider with Cleaner

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13837/files
  - new: https://git.openjdk.org/jdk/pull/13837/files/3a139b27..c08c98f3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13837&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13837&range=02-03

  Stats: 331463 lines in 3383 files changed: 281976 ins; 27082 del; 22405 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