2.x: fix Observable.combineLatest to dispose eagerly

[akarnokd]

This PR fixes Observable.combineLatest to dispose the sources outside the serialization loop, just like Flowable.combineLatest does. This allows cancellation even if the serialization loop is busy/blocking inside an onNext emission.

In addition, a unit test was added to Flowable.combineLatest as well.

Reported in #5111.

[codecov]

Codecov Report

Merging #5114 into 2.x will decrease coverage by -0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5114      +/-   ##
============================================
- Coverage     95.15%   95.09%   -0.07%     
- Complexity     5561     5565       +4     
============================================
  Files           620      620              
  Lines         40011    40014       +3     
  Branches       5632     5632              
============================================
- Hits          38072    38050      -22     
- Misses          920      944      +24     
- Partials       1019     1020       +1

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableCombineLatest.java	98.75% <100%> (+0.02%)	6 <ø> (ø)	❌
.../operators/observable/ObservableFlatMapSingle.java	89.55% <ø> (-7.47%)	2% <ø> (ø)
...internal/disposables/ArrayCompositeDisposable.java	92.85% <ø> (-7.15%)	15% <ø> (-1%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <ø> (-5.7%)	55% <ø> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <ø> (-5.13%)	14% <ø> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	87.38% <ø> (-4.21%)	56% <ø> (+1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <ø> (-2.78%)	2% <ø> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <ø> (-2.78%)	2% <ø> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.15% <ø> (-2.62%)	2% <ø> (ø)
...activex/internal/schedulers/ExecutorScheduler.java	94.07% <ø> (-2.23%)	9% <ø> (ø)
... and 38 more

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