2.x: improve the parallel() mode test coverage, improve its code

[akarnokd]

This PR adds additional unit tests for the parallel() operators, plus:

• adds conditional fusion to runOn, filter and map operators
• exposes the FlowableFlatMap's internal Subscriber to be reused with ParallelFlowable.flatMap
• uses the FlowableConcatMap's internal Subscriber to be reused with ParallelFlowable.concatMap
• fix generics with collect and reduce
• change queue overflow errors to MissingBackpressureException
• make sure join-like operators don't emit the same upstream Throwable to the RxJavaPlugins.onError handler if all rails have the same error reference

[codecov-io]

Current coverage is 95.60% (diff: 95.91%)

Merging #5006 into 2.x will increase coverage by 0.80%

@@                2.x      #5006   diff @@
==========================================
  Files           609        609          
  Lines         39228      39363   +135   
  Methods           0          0          
  Messages          0          0          
  Branches       5993       6025    +32   
==========================================
+ Hits          37188      37632   +444   
+ Misses          991        749   -242   
+ Partials       1049        982    -67   

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