2.x: test sync and missing operators (8/02)

[akarnokd]

• Add rebatchRequests which delegates to observeOn with the help of an internal ImmediateThinScheduler that only executes tasks immediately. Still not planning to expose an immediate scheduler.
• Fix FlowableMapNotification's type signature, no need to restrict it to return Publisher<R> but can go with R itself.
• Make sure flatMap doesn't reorder scalars and elements of the same inner source.
• Change parameter ordering on mergeDelayError(Iterator) overloads.

[codecov-io]

Current coverage is 69.68% (diff: 78.78%)

Merging #4273 into 2.x will increase coverage by 0.14%

@@                2.x      #4273   diff @@
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  Files           418        419     +1   
  Lines         30608      30632    +24   
  Methods           0          0          
  Messages          0          0          
  Branches       4933       4935     +2   
==========================================
+ Hits          21283      21345    +62   
+ Misses         7235       7201    -34   
+ Partials       2090       2086     -4   

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