Few WorkManager fields are not MT safe

While debugging https://github.com/eclipse-jdt/eclipse.jdt.core/issues/4370 I've noticed that in the `org.eclipse.core.internal.resources.WorkManager` few fields are not volatile/atomic but could be accessed from different threads, so even if the modification happens while holding workspace locks, other threads might see stale values.

Fields in question are
- `WorkManager.operationCanceled`
- `WorkManager.nestedOperations`
- `WorkManager.preparedOperations`

I don't have reproducer, but I think making the fields either volatile or atomic shouldn't harm.