1.x: add Single.merge(Obs), Obs.flatMapSingle & flatMapCompletable

[akarnokd]

This PR adds specialized operators for merging Single and Completable sources:

• Single.merge(Observable<Single<T>>) + maxConcurrency overload
• Single.mergeDelayError(Observable<Single<T>>) + maxConcurrency overload
• Observable.flatMapSingle(Func1<T, Single<R>>) + delayErrors + delayErrors, maxConcurrency overloads
• Observable.flatMapCompletable(Func1<T, Completable>) + delayErrors + delayErrors, maxConcurrency overloads

All of these return Observable.

This replaces #4988.

Sidenote: for some time now, contributor(s) from certain party tend to contribute low quality or inadequate code and "disappear" pretty quickly. I can't be sure this is just lazyness or a deliberate and elaborate scheme to extract free work out of me for their own party's benefit but it is annoying as hell.

[codecov]

Codecov Report

Merging #5092 into 1.x will increase coverage by 0.17%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##                1.x    #5092      +/-   ##
============================================
+ Coverage     84.12%   84.29%   +0.17%     
- Complexity     2852     2874      +22     
============================================
  Files           288      290       +2     
  Lines         17818    18070     +252     
  Branches       2420     2468      +48     
============================================
+ Hits          14989    15233     +244     
  Misses         1968     1968              
- Partials        861      869       +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/Observable.java	99.45% <100%> (ø)	448 <6> (+6)	✅
src/main/java/rx/Single.java	70.56% <100%> (+0.48%)	81 <4> (+4)	✅
...x/internal/operators/OnSubscribeFlatMapSingle.java	92.3% <92.3%> (ø)	4 <4> (?)
...ernal/operators/OnSubscribeFlatMapCompletable.java	93.02% <93.02%> (ø)	4 <4> (?)
...ava/rx/internal/operators/OperatorMaterialize.java	85.24% <ø> (-3.28%)	3% <ø> (ø)
...n/java/rx/subscriptions/CompositeSubscription.java	74.02% <ø> (-2.6%)	23% <ø> (-1%)
.../rx/internal/schedulers/CachedThreadScheduler.java	87.37% <ø> (-1.95%)	6% <ø> (ø)
...main/java/rx/internal/operators/OperatorMerge.java	86.83% <ø> (+0.23%)	7% <ø> (ø)	❌
... and 4 more

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