2.x: make parallel() a fusion-async-boundary

[akarnokd]

The parallel() supports front-fusion but since the operator is almost always followed by the separate runOn operator, the parallel version of the observeOn operator, such front-fusion should be considered a BOUNDARY-type fusion just like with observeOn.

A requestFusion with BOUNDARY tells the upstream operator(s) that when fused, their actions would be executed behind an async boundary and possibly on an unwanted thread. Operators, such as map and doOnNext, who are generally expected to be thread-confined, can then refuse to fuse, restoring the traditional queue hopping behavior (source -> queue -> op -> queue -> op -> queue -> consumer).

Reported in #5676.

/cc @smaldini & @simonbasle

[codecov]

Codecov Report

Merging #5677 into 2.x will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5677      +/-   ##
============================================
- Coverage     96.22%   96.21%   -0.01%     
+ Complexity     5813     5812       -1     
============================================
  Files           633      633              
  Lines         41552    41552              
  Branches       5751     5751              
============================================
- Hits          39983    39980       -3     
- Misses          620      629       +9     
+ Partials        949      943       -6

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/parallel/ParallelFromPublisher.java	94.31% <100%> (-1.43%)	4 <0> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.97% <0%> (-5.73%)	57% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	89.38% <0%> (-4.87%)	61% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.27% <0%> (-3.39%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...java/io/reactivex/subjects/CompletableSubject.java	97.4% <0%> (-2.6%)	36% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.27% <0%> (-1.73%)	45% <0%> (-1%)
... and 27 more

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