3.x: Add concatMap with Scheduler guaranteeing where the mapper runs

[akarnokd]

This PR adds an overload to concatMap and concatMapDelayError in both Flowable and Observable which makes sure the mapper function is always executed on a dedicated scheduler.

By default concatMap may execute the mapper function on various threads: the subscribing thread, the upstream thread generating the items to be mapped or the thread where the generated inner source terminates. These overloads remove this kind of uncertainty.

The implementation is a copy of the original concatMap instead of an extension so that optimizations for the non-scheduled variants still work without extra overhead.

Overloads to the concatMap{Single|Maybe|Completable} and concatMap{Single|Maybe|Completable}DelayError may be added in a separate PR later.

Resolves #6447

[codecov]

Codecov Report

Merging #6538 into 3.x will increase coverage by 0.07%.
The diff coverage is 98.86%.

@@             Coverage Diff             @@
##               3.x    #6538      +/-   ##
===========================================
+ Coverage     98.2%   98.27%   +0.07%     
- Complexity    6198     6214      +16     
===========================================
  Files          680      682       +2     
  Lines        44976    45505     +529     
  Branches      6202     6288      +86     
===========================================
+ Hits         44168    44722     +554     
+ Misses         259      248      -11     
+ Partials       549      535      -14

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	575 <3> (+3)	⬆️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	550 <3> (+3)	⬆️
...ators/observable/ObservableConcatMapScheduler.java	98.82% <98.82%> (ø)	4 <4> (?)
...operators/flowable/FlowableConcatMapScheduler.java	98.83% <98.83%> (ø)	4 <4> (?)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.36% <0%> (-4.26%)	9% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
... and 25 more

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