3.x: Fix Flowable.switchMap not canceling properly during onNext-cancel races

[akarnokd]

There was an unnecessary active.lazySet(null) which prevented cancellation of the inner source under some circumstances.

More specifically, when one thread issued a cancel, the cancelled flag was set, then another thread in drain would loop around, see the cancelled flag and clear the reference. Back in the cancelling thread, the disposeInner would only see null and do nothing.

Observable.switchMap did not have this mistake. Both received unit tests to verify the correct behavior.

2.x will be fixed in a separate PR.

Resolves #6914

[codecov]

Codecov Report

Merging #6917 into 3.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6917      +/-   ##
============================================
- Coverage     99.65%   99.63%   -0.03%     
+ Complexity     6670     6666       -4     
============================================
  Files           742      742              
  Lines         47217    47216       -1     
  Branches       6367     6367              
============================================
- Hits          47055    47043      -12     
- Misses           48       57       +9     
- Partials        114      116       +2     

Impacted Files	Coverage Δ	Complexity Δ
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	93.57% <0.00%> (-4.59%)	2.00% <0.00%> (ø%)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0.00%> (-2.78%)	56.00% <0.00%> (-2.00%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (-2.33%)	2.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFromIterable.java	96.87% <0.00%> (-2.09%)	5.00% <0.00%> (ø%)
.../internal/disposables/ListCompositeDisposable.java	98.00% <0.00%> (-2.00%)	34.00% <0.00%> (-1.00%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableReplay.java	99.13% <0.00%> (-0.87%)	22.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowableGroupBy.java	99.05% <0.00%> (-0.32%)	3.00% <0.00%> (ø%)
... and 9 more

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