2.x: Fix flatMap inner fused poll crash not cancelling the upstream

[akarnokd]

This PR fixes the lack of upstream cancel() call when an inner, fused source's queue.poll() crashes in a non-delayed error mode.

Unit tests were added to verify Observable.flatMap, Flowable.flatMapIterable and Observable.flatMapIterable as well.

Fixes #5791

[codecov]

Codecov Report

Merging #5792 into 2.x will decrease coverage by 0.03%.
The diff coverage is 90%.

@@             Coverage Diff              @@
##                2.x    #5792      +/-   ##
============================================
- Coverage     96.29%   96.26%   -0.04%     
+ Complexity     5809     5806       -3     
============================================
  Files           634      634              
  Lines         41607    41609       +2     
  Branches       5770     5771       +1     
============================================
- Hits          40066    40055      -11     
+ Misses          614      612       -2     
- Partials        927      942      +15

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableFlatMap.java	89.47% <90%> (+1.11%)	4 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-3.51%)	10% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.99%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../main/java/io/reactivex/subjects/MaybeSubject.java	97.75% <0%> (-2.25%)	45% <0%> (-1%)
... and 21 more

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[artem-zinnatullin]

So this is bug fix, but what's the reason for renaming s to upstream?

It looks not important to revert but I guess minimizing fix PRs saves time for those who review them while going through changelog :)

[akarnokd]

It reads nicer now.