[crac] RFR: DNS cache maintenance

[Radim Vansa]

This draft PR contains a backport of issue that prevents compilation on systems with newer glibc versions; ideally the backport should be integrated first and then this PR can be rebased.

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Commit messages:
 - Address review comments
 - Prevent Resource being garbage-collected
 - DNS cache maintainance

Changes: https://git.openjdk.org/crac/pull/40/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=40&range=00
  Stats: 251 lines in 3 files changed: 251 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/crac/pull/40.diff
  Fetch: git fetch https://git.openjdk.org/crac pull/40/head:pull/40

PR: https://git.openjdk.org/crac/pull/40