2.x: switch to throwing Action, switchMapDelayError, other cleanup

[akarnokd]

Notable changes:

• Replace the use of Runnable in the base reactive types to Action whose run method can throw a checked exception.
• Fix javadoc referencing 1.x types no longer available.
• The switchMap operator has been enhanced to support delaying errors.
• Update concat(Iterable) to use concatMapDelayError because Iterable can't throw just anytime but on the boundary, reducing the per-element overhead.
• FlowableEmitter has been enhanced with a serialize() method that serializes calls to onXXX methods.
• Factored out FlowableEmitter.Cancellable into io.reactivex.functions as it will be used by the other base reactive types with their XEmitter implementations.
• Added AtomicThrowable with convenience methods that use ExceptionHelper's terminal atomics with Throwables.

[codecov-io]

Current coverage is 68.65% (diff: 44.48%)

Merging #4357 into 2.x will decrease coverage by 0.21%

@@                2.x      #4357   diff @@
==========================================
  Files           432        434     +2   
  Lines         30815      30952   +137   
  Methods           0          0          
  Messages          0          0          
  Branches       4936       4968    +32   
==========================================
+ Hits          21224      21251    +27   
- Misses         7524       7615    +91   
- Partials       2067       2086    +19   

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[JakeWharton]

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