2.x: fix Flowable.flatMapMaybe/Single maxConcurrency not requesting more

[akarnokd]

This PR fixes Flowable.flatMapMaybe and Flowable.flatMapSingle not replenishing from the upstream when they complete/error per inner source.

[codecov]

Codecov Report

Merging #5287 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5287      +/-   ##
============================================
- Coverage     96.05%   96.03%   -0.03%     
- Complexity     5748     5755       +7     
============================================
  Files           628      628              
  Lines         41077    41085       +8     
  Branches       5699     5703       +4     
============================================
- Hits          39455    39454       -1     
- Misses          648      650       +2     
- Partials        974      981       +7

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/flowable/FlowableFlatMapSingle.java	97.82% <100%> (+2.22%)	2 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.13% <100%> (+2.1%)	2 <0> (ø)	⬇️
...a/io/reactivex/processors/SerializedProcessor.java	91.48% <0%> (-6.39%)	27% <0%> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	87.61% <0%> (-4.87%)	61% <0%> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	91.17% <0%> (-4.42%)	3% <0%> (ø)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	90.9% <0%> (-3.21%)	5% <0%> (ø)
... and 42 more

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