2.x: Fix Observable.concatMapSingle dropping upstream items

[akarnokd]

The internal queue of Observable.concatMapSingle was incorrectly the bounded one from its Flowable counterpart, causing it to drop upstream items if the current Single was delayed. The right queue for Observables is the SpscLinkedArrayQueue.

Added unit tests to both concatMapSingle and concatMapMaybe to verify the correct behavior.

Fixes: #5971.

[codecov]

Codecov Report

Merging #5972 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5972      +/-   ##
============================================
+ Coverage     98.24%   98.27%   +0.03%     
+ Complexity     6018     6017       -1     
============================================
  Files           656      656              
  Lines         44037    44037              
  Branches       6100     6100              
============================================
+ Hits          43262    43279      +17     
+ Misses          233      229       -4     
+ Partials        542      529      -13

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/mixed/ObservableConcatMapSingle.java	100% <100%> (ø)	3 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-1.39%)	56% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	98.2% <0%> (-0.9%)	59% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	86.58% <0%> (-0.64%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	89.85% <0%> (-0.49%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	98.72% <0%> (-0.43%)	11% <0%> (ø)
... and 17 more

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