2.x: Add Flowable.switchMap{Maybe,Single}{DelayError} operators

[akarnokd]

This PR adds Flowable operators that allow switching between a mapped sequence of Maybes or Singles, with or without delaying their errors:

• switchMapMaybe
• switchMapMaybeDelayError
• switchMapSingle
• switchMapSingleDelayError

They are in the same PR as the Single variant's implementation is practically the Maybe implementation minus the onComplete case.

Their marbles will be updated in a separate PR.

Originally requested in #4853.

[codecov]

Codecov Report

Merging #5873 into 2.x will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5873      +/-   ##
============================================
+ Coverage     96.57%   96.62%   +0.05%     
- Complexity     5881     5893      +12     
============================================
  Files           650      652       +2     
  Lines         43156    43403     +247     
  Branches       5994     6033      +39     
============================================
+ Hits          41677    41938     +261     
+ Misses          567      557      -10     
+ Partials        912      908       -4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	557 <5> (+4)	⬆️
...ernal/operators/mixed/FlowableSwitchMapSingle.java	100% <100%> (ø)	2 <2> (?)
...ternal/operators/mixed/FlowableSwitchMapMaybe.java	100% <100%> (ø)	2 <2> (?)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-5.32%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
... and 31 more

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[vanniktech]

Which other operators are still missing?

[akarnokd]

1. Observable.concatMapCompletableDelayError
2. Observable.concatMapMaybe
3. Observable.concatMapMaybeDelayError
4. Observable.concatMapSingle
5. Observable.concatMapSingleDelayError
6. Observable.switchMapCompletable
7. Observable.switchMapCompletableDelayError
8. Observable.switchMapMaybe
9. Observable.switchMapMaybeDelayError
10. Observable.switchMapSingle - dedicated implementation
11. Observable.switchMapSingleDelayError - dedicated implementation

These can be derived easily from their Flowable counterparts.