2.x UnicastProcessor fail-fast support

[mostroverkhov]

This PR adds support for fail-fast behavior to UnicastProcessor with methods UnicastProcessor<T> create(boolean delayError), UnicastProcessor<T> create(int capacityHint, Runnable onTerminated, boolean delayError). Relates to #5165, #5217

[vanniktech]

nit: newline between onCancelled,delayError

[akarnokd]

Space.

[vanniktech]

Is this one really necessary?

[mostroverkhov]

could you elaborate a bit?

[codecov]

Codecov Report

Merging #5226 into 2.x will increase coverage by 0.06%.
The diff coverage is 93.75%.

@@             Coverage Diff              @@
##                2.x    #5226      +/-   ##
============================================
+ Coverage     96.01%   96.07%   +0.06%     
- Complexity     5747     5758      +11     
============================================
  Files           628      628              
  Lines         41085    41099      +14     
  Branches       5698     5703       +5     
============================================
+ Hits          39446    39485      +39     
+ Misses          657      642      -15     
+ Partials        982      972      -10

Impacted Files	Coverage Δ	Complexity Δ
...java/io/reactivex/processors/UnicastProcessor.java	95.9% <93.75%> (-0.91%)	64 <6> (+7)
...a/io/reactivex/processors/SerializedProcessor.java	91.48% <0%> (-6.39%)	27% <0%> (-1%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <0%> (-6.13%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.04% <0%> (-4.48%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ternal/operators/completable/CompletableCache.java	96.96% <0%> (-3.04%)	23% <0%> (-1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
... and 40 more

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[akarnokd]

One of the new package-private constructors is unnecessary.

[akarnokd]

This is unnecessary.

[mostroverkhov]

3 args constructor has non-null onTerminate check; should i move this check to factory?

[akarnokd]

Could you move that check into the appropriate create method(s)?