RFR: 8330815: Use pattern matching for instanceof in KeepAliveCache

[Christoph Langer]

In su.net.www.http.KeepAliveCache we could use pattern matching for instanceof.

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Commit messages:
 - JDK-8330815

Changes: https://git.openjdk.org/jdk/pull/18885/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18885&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330815
  Stats: 9 lines in 1 file changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18885.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18885/head:pull/18885

PR: https://git.openjdk.org/jdk/pull/18885