2.x: test sync from Observable to Flowable 10/16-1

[akarnokd]

• copy the recent unit tests from Observable to Flowable
• fix the same bugs
• perform the same cleanups
• compact the same operators
• factor out fusion method and constants to QueueFuseable
• have Flowable.toList(Callable) return Single

[codecov-io]

Current coverage is 89.08% (diff: 86.53%)

Merging #4714 into 2.x will increase coverage by 1.44%

@@                2.x      #4714   diff @@
==========================================
  Files           569        570     +1   
  Lines         37124      37268   +144   
  Methods           0          0          
  Messages          0          0          
  Branches       5655       5685    +30   
==========================================
+ Hits          32536      33199   +663   
+ Misses         2910       2476   -434   
+ Partials       1678       1593    -85   

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