RFR: 8378872: Mark waitList in FetcherInfo final

[Alexey Ivanov]

The `FetcherInfo` class stores information for the `ImageFetcher` class. The `waitList` field of `FetcherInfo` is used in synchronised blocks, but it's not marked as `final`.

If the reference in `waitList` changes, it could break the synchronisation.

To avoid uncertainties, mark the `waitList` field final.

Additionally, the `fetchers` array should also be `final`, the reference is never changed.

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Commit messages:
 - 8378872: Mark waitList in FetcherInfo final

Changes: https://git.openjdk.org/jdk/pull/29966/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29966&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378872
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29966.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29966/head:pull/29966

PR: https://git.openjdk.org/jdk/pull/29966