2.x: flatMap{Completable, Maybe, Single} operators

[akarnokd]

This PR adds Observable.flatMapCompletable(Function<T, CompletableSource>) and Flowable.flatMapCompletable(Function<T, CompletableSource>), both resulting in Completable (+ support for fusing back to the original type).

Edit

Added flatMapMaybe and flatMapSingle variants to both Flowable and Observable.

[akarnokd]

The build gets killed at the jacocoTestResults:

:jacocoTestReportgradle/buildViaTravis.sh: line 16:  2970 Killed                  ./gradlew -Prelease.useLastTag=true build

I've rerun it a couple of times. Maybe Travis is overloaded at the moment.

[codecov-io]

Current coverage is 82.13% (diff: 72.71%)

Merging #4667 into 2.x will decrease coverage by 0.21%

@@                2.x      #4667   diff @@
==========================================
  Files           557        565     +8   
  Lines         36402      37399   +997   
  Methods           0          0          
  Messages          0          0          
  Branches       5568       5746   +178   
==========================================
+ Hits          29974      30716   +742   
- Misses         4437       4607   +170   
- Partials       1991       2076    +85   

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[akarnokd]

Updated with the remaining operators.

[JakeWharton]

My brain melted slightly reviewing these operators, but looks good.

[vanniktech]

Thanks a lot for taking this one over @akarnokd