2.x: fix flatMap not cancelling the upstream eagerly

[akarnokd]

This PR fixes the lack of eager cancellation when flatmapping sources and not stopping the upstream if the inner source fails.

Unit tests were added to verify Single (in case it receives a dedicated implementation one day as currently it delegates to Flowable), Flowable and Observable.

Reported in #5132.

[codecov]

Codecov Report

Merging #5133 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5133      +/-   ##
============================================
+ Coverage     95.92%   95.94%   +0.02%     
+ Complexity     5608     5601       -7     
============================================
  Files           620      620              
  Lines         39810    39820      +10     
  Branches       5597     5599       +2     
============================================
+ Hits          38186    38207      +21     
+ Misses          651      640      -11     
  Partials        973      973

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableFlatMap.java	88% <100%> (+0.05%)	4 <ø> (ø)	❌
...a/io/reactivex/processors/SerializedProcessor.java	91.48% <ø> (-6.39%)	27% <ø> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <ø> (-6.25%)	2% <ø> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <ø> (-6.13%)	2% <ø> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <ø> (-5.68%)	31% <ø> (-4%)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <ø> (-4.48%)	2% <ø> (ø)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <ø> (-4.35%)	46% <ø> (-2%)
...ernal/operators/flowable/FlowableFromIterable.java	91.97% <ø> (-3.21%)	5% <ø> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <ø> (-2.78%)	2% <ø> (ø)
...nternal/operators/observable/ObservableCreate.java	97.32% <ø> (-2.68%)	2% <ø> (ø)
... and 36 more

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