[crac] RFR: Linux file system watcher support

[joeylee]

`inotify` monitors changes on filesystem, this support automatic restore for LinuxFileWatcher.

FileWatcherAfterRestoreTest verifies watcher service works after restore.
FileWatcherTest verifies automatic closing inotiify fd

The watcher keys are still managed by user, so exception will be thrown if no watcher keys are leaked, as in FileWatcherWithOpenKeysTest

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Commit messages:
 - Linux file system watcher support

Changes: https://git.openjdk.org/crac/pull/71/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=71&range=00
  Stats: 345 lines in 4 files changed: 314 ins; 25 del; 6 mod
  Patch: https://git.openjdk.org/crac/pull/71.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/71/head:pull/71

PR: https://git.openjdk.org/crac/pull/71