3.x: Fix Single.timeout race condition

[akarnokd]

The PR fixes a race condition in Single.timeout where when the time was really small, the timeout logic in run would race with the onSubscribe logic and not handle the the timeout case.

The race happened between Disposable d = get() and the later compareAndSet(d, DisposableHelper.DISPOSED). When the get executes, it sees null. Then comes onSubscribe and atomically sets some Disposable in this. Now compareAndSet fails because the null -> Disposed transition can't be made.

The fix is to use DisposableHelper.dispose which returns true iff the current thread managed to transition from a non-disposed state into a disposed state.

Fixes #7514

[codecov]

Codecov Report

Merging #7515 (12f20fb) into 3.x (e1b6cb4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7515      +/-   ##
============================================
+ Coverage     99.53%   99.55%   +0.02%     
  Complexity     6798     6798              
============================================
  Files           752      752              
  Lines         47673    47670       -3     
  Branches       6400     6399       -1     
============================================
+ Hits          47452    47460       +8     
+ Misses          101       96       -5     
+ Partials        120      114       -6     

Impacted Files	Coverage Δ
...java3/internal/operators/single/SingleTimeout.java	100.00% <100.00%> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.90% <0.00%> (-6.43%)	⬇️
...nternal/operators/observable/ObservableCreate.java	95.72% <0.00%> (-1.71%)	⬇️
...ternal/operators/completable/CompletableMerge.java	98.64% <0.00%> (ø)
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	⬆️
...3/internal/operators/flowable/FlowablePublish.java	100.00% <0.00%> (+0.99%)	⬆️
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (+1.16%)	⬆️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	97.92% <0.00%> (+2.07%)	⬆️
.../operators/observable/ObservableFlatMapSingle.java	97.61% <0.00%> (+3.17%)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	97.88% <0.00%> (+4.92%)	⬆️

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