2.x: fix Observable.zip to dispose eagerly

[jbarr21]

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

In addition, a unit test was added to Observable.zip as well.

Reported in #5111.

[codecov-io]

Codecov Report

Merging #5121 into 2.x will decrease coverage by -0.07%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #5121      +/-   ##
===========================================
- Coverage     95.17%   95.1%   -0.07%     
+ Complexity     5566    5564       -2     
===========================================
  Files           620     620              
  Lines         40014   40020       +6     
  Branches       5632    5633       +1     
===========================================
- Hits          38082   38062      -20     
- Misses          929     941      +12     
- Partials       1003    1017      +14

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/observable/ObservableZip.java	100% <100%> (ø)	6 <ø> (ø)	❌
...al/operators/observable/ObservableSampleTimed.java	90% <ø> (-6.67%)	3% <ø> (ø)
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <ø> (-6.67%)	17% <ø> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <ø> (-5.23%)	2% <ø> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <ø> (-4.09%)	2% <ø> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <ø> (-3.58%)	2% <ø> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.75% <ø> (-2.18%)	2% <ø> (ø)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <ø> (-2.18%)	46% <ø> (-1%)
...operators/observable/ObservableConcatMapEager.java	97.29% <ø> (-2.17%)	2% <ø> (ø)
...activex/internal/disposables/DisposableHelper.java	98.14% <ø> (-1.86%)	27% <ø> (-1%)
... and 28 more

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