2.x: reflection-based parameter validator & fixes

[akarnokd]

This PR adds a reflection-based parameter validator to check the base reactive classes for proper parameter validation: null checks and argument ranges. The problems detected are fixed as well.

[codecov]

Codecov Report

Merging #5187 into 2.x will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5187      +/-   ##
============================================
+ Coverage     95.98%   96.04%   +0.05%     
- Complexity     5665     5673       +8     
============================================
  Files           621      621              
  Lines         40340    40581     +241     
  Branches       5616     5620       +4     
============================================
+ Hits          38722    38975     +253     
- Misses          638      641       +3     
+ Partials        980      965      -15

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	99.32% <100%> (+0.02%)	131 <12> (ø)	❌
.../operators/flowable/FlowableBlockingSubscribe.java	97.22% <100%> (+0.25%)	10 <1> (ø)	❌
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	522 <19> (ø)	❌
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	506 <46> (ø)	❌
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	168 <10> (+1)	✅
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	112 <1> (ø)	❌
...rators/observable/ObservableBlockingSubscribe.java	96.96% <100%> (+0.3%)	9 <1> (ø)	❌
.../java/io/reactivex/subscribers/TestSubscriber.java	99.27% <100%> (+0.01%)	57 <0> (+1)	✅
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-6.56%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
... and 35 more

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[vanniktech]

Nice 👍

[akarnokd]

Thanks @vanniktech ! I'm merging this because there are a couple of inbound API changes from other contributors and having this in place avoids conflicts.