2.x: Improve the scalar source performance of Observable.(concat|switch)MapX

[akarnokd]

This PR adds scalar source optimizations to Observable operators:

• concatMapCompletable
• concatMapSingle
• concatMapMaybe
• switchMapCompletable
• switchMapSingle
• switchMapMaybe

Benchmark

i7 4770K, Windows 7 x64, Java 8u162

The baseline is taken from #5914. The target is to be faster than using the plain concatMap or switchMap with a toObservable conversion. The apparent shortcomings of longer concatMapMaybe and concatMapSingle will be addressed in a subsequent PR.

[codecov]

Codecov Report

Merging #5918 into 2.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5918      +/-   ##
============================================
+ Coverage     98.19%   98.24%   +0.04%     
- Complexity     6002     6020      +18     
============================================
  Files           655      656       +1     
  Lines         43963    44004      +41     
  Branches       6090     6094       +4     
============================================
+ Hits          43169    43231      +62     
+ Misses          243      228      -15     
+ Partials        551      545       -6

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/mixed/ObservableSwitchMapMaybe.java	100% <100%> (ø)	3 <2> (+1)	⬆️
...perators/mixed/ObservableConcatMapCompletable.java	99.24% <100%> (+0.59%)	3 <2> (-4)	⬇️
...ex/internal/operators/maybe/MaybeToObservable.java	100% <100%> (ø)	4 <2> (+1)	⬆️
...rnal/operators/mixed/ObservableConcatMapMaybe.java	100% <100%> (ø)	3 <2> (+1)	⬆️
.../internal/operators/single/SingleToObservable.java	100% <100%> (ø)	3 <2> (+1)	⬆️
...nal/operators/mixed/ObservableConcatMapSingle.java	100% <100%> (ø)	3 <2> (+1)	⬆️
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <100%> (+0.01%)	3 <2> (+1)	⬆️
...nal/operators/mixed/ObservableSwitchMapSingle.java	100% <100%> (ø)	3 <2> (+1)	⬆️
...ex/internal/operators/mixed/ScalarXMapZHelper.java	100% <100%> (ø)	13 <13> (?)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
... and 27 more

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