RFR: 8325179: Race in BasicDirectoryModel.validateFileCache

[Alexey Ivanov]

Ensure access to the `filesLoader` field of `BasicDirectoryModel` is synchronized.

Without synchronization, a thread checks if `filesLoader` is not null and creates a new `FilesLoader` thread. If the thread is pre-empted between these two operations, another thread or even several threads can see the `null` value and create new `FilesLoader` threads.

The same way, `BasicDirectoryModel.invalidateFileCache` needs to be synchornized to interrupt the current `filesLoader` and assign `null`.

This bug allows reproducing `ConcurrentModificationException` seen in [JDK-8323670](https://bugs.openjdk.org/browse/JDK-8323670) and [JDK-8307091](https://bugs.openjdk.org/browse/JDK-8307091) using the test in PR #18109.

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Commit messages:
 - Remove volatile modifier from DoChangeContents in FilesLoader
 - Remove synchronized modifier from DoChangeContents.run
 - Declare addStart and remStart in DoChangeContents as final
 - Reverse condition in DoChangeContents.run to decrease indentation
 - Remove cancelRunnables
 - Declare newFiles as final
 - Remove redundant fileSystem variable in FilesLoader.run0
 - Create FilesLoader inside synchronized section

Changes: https://git.openjdk.org/jdk/pull/18111/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325179
  Stats: 61 lines in 1 file changed: 14 ins; 23 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/18111.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18111/head:pull/18111

PR: https://git.openjdk.org/jdk/pull/18111