[jdk11u-dev] RFR: 8325179: Race in BasicDirectoryModel.validateFileCache

[Martin Doerr]

Clean backport of [JDK-8325179](https://bugs.openjdk.org/browse/JDK-8325179) and [JDK-8238169](https://bugs.openjdk.org/browse/JDK-8238169) which belong together.

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Commit messages:
 - Backport e66788c16563d343f6cccd2807a251ccc6f9b64a

Changes: https://git.openjdk.org/jdk11u-dev/pull/2768/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2768&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325179
  Stats: 69 lines in 1 file changed: 17 ins; 21 del; 31 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2768.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2768/head:pull/2768

PR: https://git.openjdk.org/jdk11u-dev/pull/2768