2.x: add parallel hooks to RxJavaPlugins, add missing params validation

[akarnokd]

This PR adds an onAssembly hook for ParallelFlowable operators and adds the missing parameter validation to some parallel operators.

[codecov-io]

Codecov Report

Merging #5043 into 2.x will increase coverage by <.01%.

@@             Coverage Diff              @@
##                2.x    #5043      +/-   ##
============================================
+ Coverage     95.63%   95.63%   +<.01%     
- Complexity     5539     5542       +3     
============================================
  Files           612      612              
  Lines         39545    39576      +31     
  Branches       5551     5553       +2     
============================================
+ Hits          37817    37850      +33     
+ Misses          748      747       -1     
+ Partials        980      979       -1

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/plugins/RxJavaPlugins.java	100% <100%> (+0.72%)	138 <5> (+5)	✅
...n/java/io/reactivex/parallel/ParallelFlowable.java	100% <100%> (ø)	41 <16> (ø)	❌
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.44% <ø> (-4.21%)	55% <ø> (+1%)
...internal/operators/completable/CompletableAmb.java	94.44% <ø> (-3.71%)	10% <ø> (-1%)
.../main/java/io/reactivex/subjects/MaybeSubject.java	97.82% <ø> (-2.18%)	47% <ø> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	97.94% <ø> (-2.06%)	3% <ø> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	97.95% <ø> (-2.05%)	2% <ø> (ø)
...nternal/operators/observable/ObservableCreate.java	97.32% <ø> (-1.79%)	2% <ø> (ø)
...n/java/io/reactivex/processors/AsyncProcessor.java	97.45% <ø> (-1.7%)	55% <ø> (-1%)
... and 36 more

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