2.x: add ParallelFlowable.sequentialDelayError

[akarnokd]

This PR adds the sequentialDelayError operator to ParallelFlowable that allows awaiting all 'rails' in a parallel flow to terminate normally or with the (composite) exception of the failed rail(s).

To enable this, the Flowable.parallel() operator's behavior regarding rail cancellation had to be changed. In v2.0.5 if any of the rails cancelled (maybe due to a failure, maybe due to an end consumer cancelling the entire parallel flow) the input Flowable was cancelled.

This PR alters this by requiring all rails to cancel before cancelling the input Flowable. The change permits one or multiple rails to fail and let others progress in case the new sequentialDelayError is applied as a terminal operator. The original sequential() operator still cancels all rails if one of them fails (triggering the cancellation of the input Flowable).

Note that this change may still drop and never process elements in the internal queues of the operators in the parallel flow (because the parallel processing is not implemented with work-stealing that could pick up elements from a dead queue). In order to get as many elements processed as possible, it is recommended to reduce the default prefetch on the runOn operator to a reasonable tradeoff value (between throughput and fault tolerance).

Edit

Updated the PR to make sure parallel() ignores cancelled rails when it dispatches items.

Related: #5108.

[codecov-io]

Codecov Report

Merging #5117 into 2.x will decrease coverage by -0.03%.
The diff coverage is 91.03%.

@@             Coverage Diff             @@
##               2.x    #5117      +/-   ##
===========================================
- Coverage       96%   95.97%   -0.03%     
- Complexity    5598     5609      +11     
===========================================
  Files          620      620              
  Lines        39699    39810     +111     
  Branches      5567     5597      +30     
===========================================
+ Hits         38113    38208      +95     
+ Misses         629      627       -2     
- Partials       957      975      +18

Impacted Files	Coverage Δ	Complexity Δ
...n/java/io/reactivex/parallel/ParallelFlowable.java	100% <100%> (ø)	43 <2> (+2)	✅
...ivex/internal/operators/parallel/ParallelJoin.java	94.83% <90.26%> (-0.47%)	3 <3> (+1)
...rnal/operators/parallel/ParallelFromPublisher.java	95.73% <92.85%> (-0.84%)	4 <ø> (ø)
...al/operators/observable/ObservableSampleTimed.java	91.66% <ø> (-5%)	3% <ø> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <ø> (-4.09%)	2% <ø> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <ø> (-4%)	2% <ø> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <ø> (-3.78%)	24% <ø> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	91.3% <ø> (-3.63%)	2% <ø> (ø)
.../internal/operators/flowable/FlowableInterval.java	93.33% <ø> (-3.34%)	2% <ø> (ø)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	95.58% <ø> (-2.95%)	2% <ø> (ø)
... and 42 more

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