[2.x] UnicastSubject fail fast support

[mostroverkhov]

This PR adds support for fail-fast behavior to UnicastSubject with methods UnicastSubject<T> create(boolean delayError), UnicastSubject<T> create(int capacityHint, Runnable onTerminated, boolean delayError). Relates to #5165

[vanniktech]

Should this be marked Experimental?

[vanniktech]

Should this be marked Experimental?

[vanniktech]

nit: missing .

[vanniktech]

Is this one really needed? The factories could just go and call UnicastSubject(capacityHint, true) ?

[codecov]

Codecov Report

Merging #5217 into 2.x will decrease coverage by 0.04%.
The diff coverage is 97.5%.

@@             Coverage Diff              @@
##                2.x    #5217      +/-   ##
============================================
- Coverage     96.04%   95.99%   -0.05%     
- Complexity     5705     5711       +6     
============================================
  Files           624      624              
  Lines         40683    40705      +22     
  Branches       5648     5655       +7     
============================================
+ Hits          39075    39076       +1     
- Misses          632      655      +23     
+ Partials        976      974       -2

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/io/reactivex/subjects/UnicastSubject.java	98.75% <97.5%> (-0.53%)	65 <12> (+12)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <0%> (-7.78%)	55% <0%> (-2%)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.78% <0%> (-4.85%)	60% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...ctivex/internal/schedulers/AbstractDirectTask.java	95.65% <0%> (-4.35%)	15% <0%> (-1%)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-3.51%)	10% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...n/java/io/reactivex/processors/AsyncProcessor.java	95.76% <0%> (-3.39%)	54% <0%> (-2%)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
... and 42 more

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[akarnokd]

Could you also do it for UnicastProcessor?

[akarnokd]

Also please add @since 2.0.8 - experimental to all the new methods.

[mostroverkhov]

Yes, will do that as separate PR

[akarnokd]

Thanks!