2.x: Add fusion support to ObservableSwitchMap inner source

[akarnokd]

This PR improves the performance of the Observable.switchMap operator by adding fusion support as well as upgrading the Completable and Single conversion to Observable to be fuseable. The PR also fixes an error in the DeferredScalarDisposable implementation as the protocol requires calling with null.

The plain (Observable.switchMap(Observable)) conversion has promising performance improvements:

The conversion Observable.switchMap(Maybe.toObservable) has promising performance improvements:

The conversion Observable.switchMap(Completable.toObservable) is also promising, but there is a 11% loss in the scalar case for some reason:

Finally, the conversion Observable.switchMap(Single.toObservable) got a bigger hit for the scalar case, 21% loss, but the rest are impressive:

[akarnokd]

I don't know why those losses appeared. The change to ObserverCompletableObserver has only structural change but no code path change. The SingleToObservableObserver now supports fusion which means longer code path on a success value.

[codecov]

Codecov Report

Merging #5919 into 2.x will decrease coverage by <.01%.
The diff coverage is 96.42%.

@@             Coverage Diff              @@
##                2.x    #5919      +/-   ##
============================================
- Coverage     98.24%   98.24%   -0.01%     
+ Complexity     6019     6015       -4     
============================================
  Files           656      656              
  Lines         44004    44040      +36     
  Branches       6094     6102       +8     
============================================
+ Hits          43233    43265      +32     
- Misses          230      233       +3     
- Partials        541      542       +1

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/observers/DeferredScalarDisposable.java	97.91% <100%> (+0.04%)	20 <0> (ø)	⬇️
...operators/completable/CompletableToObservable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
.../internal/operators/single/SingleToObservable.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <95.45%> (+4.68%)	3 <0> (ø)	⬇️
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-4.17%)	55% <0%> (-3%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-2.66%)	11% <0%> (+1%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
... and 29 more

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