2.x: Cleanup Observable.flatMap drain logic

[akarnokd]

Cleanup the drain logic of Observable.flatMap by removing unnecessary loops and fixing the index management in case of a fused failure in one of the sources triggering another round over the same source unnecessarily.

Resolves: #6231

[codecov]

Codecov Report

Merging #6232 into 2.x will increase coverage by 0.03%.
The diff coverage is 55.55%.

@@             Coverage Diff              @@
##                2.x    #6232      +/-   ##
============================================
+ Coverage     98.22%   98.26%   +0.03%     
+ Complexity     6202     6201       -1     
============================================
  Files           667      667              
  Lines         44888    44883       -5     
  Branches       6218     6216       -2     
============================================
+ Hits          44093    44104      +11     
+ Misses          256      241      -15     
+ Partials        539      538       -1

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/observable/ObservableFlatMap.java	88.63% <55.55%> (+0.77%)	3 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	93.46% <0%> (-1.97%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...internal/operators/flowable/FlowableFromArray.java	94.91% <0%> (-1.7%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
... and 22 more

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