2.x: add withLatestFrom many, cleanups and other enhancements

[akarnokd]

Notable changes:

• implemented withLatestFrom with multiple other sources (both Flowable and Observable)
• added missing Exceptions.throwIfFatal() after catching throwables
• added SequentialDisposable and replaced internal use of SerialDisposable with it
• added package-info.java to public packages
• added javadoc to some interfaces and methods
• removed @Experimental tags and set those methods to @since 2.0
• added HalfSerializer to deal with single onNext and multiple onError/onComplete callers

[codecov-io]

Current coverage is 68.45% (diff: 62.87%)

Merging #4368 into 2.x will decrease coverage by 0.12%

@@                2.x      #4368   diff @@
==========================================
  Files           442        446     +4   
  Lines         31488      32154   +666   
  Methods           0          0          
  Messages          0          0          
  Branches       5042       5092    +50   
==========================================
+ Hits          21595      22012   +417   
- Misses         7797       8033   +236   
- Partials       2096       2109    +13   

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[JakeWharton]

👍

[akarnokd]

Thanks @JakeWharton !