2.x: fix Obs.combineLatestDelayError sync initial error not emitting

[akarnokd]

This PR fixes the case reported in #5558 where a synchronous first, non-empty source triggered an error completion immediately instead of giving a chance to the other sources to form combinations.

The fix also includes:

• initialization of the CombineObservers now happen in the constructor,
• the AtomicReference<Disposable> of the inner observers are now inlined into CombineObserver,
• cleaned up drain logic
• unit tests verifying FlowableCombineLatest (no changes required there).

[codecov]

Codecov Report

Merging #5560 into 2.x will decrease coverage by 0.16%.
The diff coverage is 93.5%.

@@             Coverage Diff              @@
##                2.x    #5560      +/-   ##
============================================
- Coverage     96.32%   96.15%   -0.17%     
+ Complexity     5830     5820      -10     
============================================
  Files           631      631              
  Lines         41414    41417       +3     
  Branches       5740     5739       -1     
============================================
- Hits          39890    39823      -67     
- Misses          596      635      +39     
- Partials        928      959      +31

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableCombineLatest.java	96.93% <93.5%> (-1.82%)	6 <0> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.97% <0%> (-5.73%)	57% <0%> (ø)
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-5.23%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	88.49% <0%> (-4.87%)	62% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...a/io/reactivex/internal/queue/MpscLinkedQueue.java	98.03% <0%> (-1.97%)	17% <0%> (-1%)
... and 19 more

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