2.x: fix LambdaObserver not cancelling the upstream

[akarnokd]

The LambdaObserver didn't cancel the upstream when its onSubscribe and onNext callbacks crashed. Also reported on the mailing list:

Dave Smith: When I am using an operator in Observable that calls subscribe (forEach as a example) and the onNext function throws an Exception the underlying class (LambdaObserver) marks the subscription disposed but does not notify upstream that is is disposed. What is the reason for this?

The LambdaSubscriber was working correctly. Both received unit tests to ensure the correct behavior.

[codecov]

Codecov Report

Merging #5170 into 2.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5170      +/-   ##
============================================
- Coverage     95.94%   95.94%   -0.01%     
+ Complexity     5658     5656       -2     
============================================
  Files           621      621              
  Lines         39972    39974       +2     
  Branches       5610     5610              
============================================
  Hits          38352    38352              
+ Misses          643      642       -1     
- Partials        977      980       +3

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	❌
...operators/observable/ObservableInternalHelper.java	86.31% <ø> (ø)	18 <0> (ø)	❌
...o/reactivex/internal/observers/LambdaObserver.java	100% <100%> (ø)	12 <0> (ø)	❌
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-8.34%)	3% <0%> (ø)
.../java/io/reactivex/subjects/SerializedSubject.java	89.47% <0%> (-6.32%)	26% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-5.89%)	2% <0%> (ø)
...ternal/operators/completable/CompletableUsing.java	95.23% <0%> (-4.77%)	4% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-3.74%)	2% <0%> (ø)
...ava/io/reactivex/internal/util/HalfSerializer.java	97.5% <0%> (-2.5%)	22% <0%> (-1%)
... and 32 more

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