2.x: Fix Observable.concatMapEager queueing of source items

[akarnokd]

This PR fixes the wrong queue type in Observable.concatMapEager to be the SpscLinkedArrayQueue instead of the SpscArrayQueue (the latter basically ignored items from the main source when it got full - a remnant from its bounded Flowable cousin). Unit tests added that ensure both Flowable and Observable variants work properly.

Reported in: #5608

[codecov]

Codecov Report

Merging #5609 into 2.x will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5609      +/-   ##
============================================
+ Coverage     96.18%   96.25%   +0.07%     
+ Complexity     5837     5836       -1     
============================================
  Files           632      632              
  Lines         41473    41473              
  Branches       5745     5745              
============================================
+ Hits          39889    39920      +31     
+ Misses          632      611      -21     
+ Partials        952      942      -10

Impacted Files	Coverage Δ	Complexity Δ
...operators/observable/ObservableConcatMapEager.java	99.45% <100%> (ø)	2 <0> (ø)	⬇️
...erators/flowable/FlowableOnBackpressureLatest.java	94.59% <0%> (-5.41%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.18%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-1.76%)	11% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-1.42%)	32% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.07%)	5% <0%> (ø)
.../operators/observable/ObservableSequenceEqual.java	98.31% <0%> (-0.85%)	2% <0%> (ø)
... and 23 more

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