3.x: Fix Observable.flatMap with maxConcurrency hangs

[akarnokd]

Queued up scalar values were not counted as completed, thus the subsequent sources where not subscribed to to fill up the allowed concurrency level.

Fixes #6945

[vanniktech]

Flowable works?

[akarnokd]

Yes. Added the test for it too anyway.

[codecov]

Codecov Report

Merging #6946 into 3.x will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #6946      +/-   ##
============================================
- Coverage     99.66%   99.60%   -0.07%     
+ Complexity     6665     6664       -1     
============================================
  Files           742      742              
  Lines         47220    47228       +8     
  Branches       6367     6369       +2     
============================================
- Hits          47064    47043      -21     
- Misses           46       60      +14     
- Partials        110      125      +15     

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/observable/ObservableFlatMap.java	98.22% <100.00%> (-0.32%)	3.00 <0.00> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-3.97%)	2.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	95.14% <0.00%> (-3.89%)	6.00% <0.00%> (ø%)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0.00%> (-2.78%)	56.00% <0.00%> (-2.00%)
...a3/internal/subscribers/InnerQueuedSubscriber.java	97.72% <0.00%> (-2.28%)	16.00% <0.00%> (-1.00%)
...ernal/operators/flowable/FlowableFromIterable.java	97.91% <0.00%> (-2.09%)	5.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	98.36% <0.00%> (-1.64%)	2.00% <0.00%> (ø%)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0.00%> (-0.91%)	2.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableCreate.java	99.14% <0.00%> (-0.86%)	2.00% <0.00%> (ø%)
... and 14 more

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