2.x: Add missing {Maybe|Single}.mergeDelayError variants

[akarnokd]

This PR adds mergeDelayError overloads to Maybe and Single; the infrastructure was there from the beginning but the delayErrors == true settings were not exposed publicly.

• Maybe.mergeDelayError(Publisher, int)
• Single.mergeDelayError(Iterable)
• Single.mergeDelayError(Publisher)
• Single.mergeDelayError(SingleSource, SingleSource)
• Single.mergeDelayError(SingleSource, SingleSource, SingleSource)
• Single.mergeDelayError(SingleSource, SingleSource, SingleSource, SingleSource)

[vanniktech]

technically it's a SingleSource. dunno whether you want to distinguish between them

[akarnokd]

I'll update it as Maybe.merge() say "a MaybeSource to be merged".

[codecov]

Codecov Report

Merging #5799 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #5799      +/-   ##
===========================================
+ Coverage     96.29%   96.3%   +<.01%     
- Complexity     5808    5812       +4     
===========================================
  Files           634     634              
  Lines         41631   41647      +16     
  Branches       5776    5776              
===========================================
+ Hits          40090   40108      +18     
+ Misses          605     597       -8     
- Partials        936     942       +6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	142 <5> (+5)	⬆️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <1> (+1)	⬆️
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.41% <0%> (-4.17%)	34% <0%> (-3%)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	95.72% <0%> (-2.57%)	2% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-1.64%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-1.31%)	2% <0%> (ø)
... and 25 more

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