3.x: [Java 8] Add Observable operators + cleanup

[akarnokd]

Add the following Java 8 operators to Observable:

• fromOptional
• fromCompletionStage
• fromStream
• firstStage
• firstOrErrorStage
• singeStage
• singleOrErrorStage
• lastStage
• lastOrErrorStage
• blockingStream
• mapOptional
• collect
• concatMapStream / flatMapStream

In addition, some validators received reporting improvements (such as using at so the IDE can jump to the exact line of the issue). Consequently, all local variable misnaming of UnicastSubject up and UnicastProcessor us have been fixed as well.

Related #6776

[akarnokd]

Marbles:

[codecov]

Codecov Report

Merging #6797 into 3.x will decrease coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6797      +/-   ##
============================================
- Coverage     98.24%   98.17%   -0.08%     
- Complexity     6283     6345      +62     
============================================
  Files           696      706      +10     
  Lines         45427    45846     +419     
  Branches       6233     6280      +47     
============================================
+ Hits          44632    45010     +378     
- Misses          278      309      +31     
- Partials        517      527      +10

Impacted Files	Coverage Δ	Complexity Δ
...a3/internal/jdk8/ObservableFirstStageObserver.java	100% <100%> (ø)	5 <5> (?)
...va3/internal/jdk8/ObservableLastStageObserver.java	100% <100%> (ø)	6 <6> (?)
...rxjava3/internal/jdk8/ObservableFlatMapStream.java	100% <100%> (ø)	5 <5> (?)
...ex/rxjava3/internal/jdk8/ObservableFromStream.java	100% <100%> (ø)	5 <5> (?)
...x/rxjava3/internal/jdk8/ObservableMapOptional.java	100% <100%> (ø)	2 <2> (?)
...ivex/rxjava3/internal/jdk8/FlowableFromStream.java	100% <100%> (+0.75%)	6 <0> (ø)	⬇️
...rxjava3/internal/jdk8/ObservableStageObserver.java	100% <100%> (ø)	9 <9> (?)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <100%> (ø)	553 <17> (+16)	⬆️
...nal/jdk8/ObservableCollectWithCollectorSingle.java	100% <100%> (ø)	3 <3> (?)
...3/internal/jdk8/ObservableSingleStageObserver.java	100% <100%> (ø)	7 <7> (?)
... and 33 more

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