2.x: Fix Observable.flatMap with maxConcurrency hangs (#6947)

[vjgarciag96]

Just a backport to 2.x of the fix for the scalar-queue max-concurrency issue already fixed for 3.x in #6946

The code for the fix has been added and also the test for it.

In addition, as was also done for #6946, the corresponding test for FlowableFlatMap has been added.

Resolves #6947

[codecov]

Codecov Report

Merging #6960 into 2.x will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                2.x    #6960   +/-   ##
=========================================
  Coverage     98.23%   98.23%           
- Complexity     6344     6346    +2     
=========================================
  Files           677      677           
  Lines         45540    45548    +8     
  Branches       6339     6341    +2     
=========================================
+ Hits          44735    44745   +10     
+ Misses          256      255    -1     
+ Partials        549      548    -1     

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/observable/ObservableFlatMap.java	90.22% <100.00%> (+1.22%)	3.00 <0.00> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-3.97%)	2.00% <0.00%> (ø%)
...perators/mixed/ObservableConcatMapCompletable.java	97.74% <0.00%> (-2.26%)	3.00% <0.00%> (ø%)
.../internal/disposables/ListCompositeDisposable.java	98.00% <0.00%> (-2.00%)	34.00% <0.00%> (-1.00%)
...l/operators/observable/ObservableFlatMapMaybe.java	85.62% <0.00%> (-1.97%)	2.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.20% <0.00%> (-1.45%)	2.00% <0.00%> (ø%)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0.00%> (-1.13%)	6.00% <0.00%> (ø%)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0.00%> (-0.86%)	2.00% <0.00%> (ø%)
...perators/single/SingleFlatMapIterableFlowable.java	97.50% <0.00%> (-0.84%)	2.00% <0.00%> (ø%)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0.00%> (-0.73%)	2.00% <0.00%> (ø%)
... and 13 more

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