This closes #587

 - Drop redundant routing actor
 - Consistently set the flag to subscribe to updates or not.
 - Scala style cleanups: Drop default values for some method parameters.

This greatly reduces compile time while still supporting the same
feature set.

[FLINK-1862] [apis] Add support for non-serializable types for collect() by switching from Java serialization to Flink serialization

Consolidate class loading tests into one cluster execution to validate repeated different class loaders

  - Accumulators rely consistently on java serialization
  - Fix accumulator transport to support custom classes
  - Fix collect() accumulator to use correct class loader

[runtime] Accumulators are reported through the RuntimeEnvironment, not directly sent as an actor message

This commit adds a figure giving an overview of Flink's stack. Users can click
on components to go to the respective component docs.

This closes #585.

This closes #563.

[quickstarts] Set different default groupid for quickstarts to avoid exclusion by the assembly plugin

This closes #544

This closes #547

[FLINK-1847] [Scala API] Changes the return type of Scala API's collect method to Seq[T] and adds parentheses to collect and count methods.

 - Updates existing test cases using Scala API's collect/count method.

This closes #583

[FLINK-1845] [runtime] Fixup ReusingKeyGroupedIterator value iterators in NonReusingSortMergeCoGroupIterator.

This closes #559

This closes #555.

Closes #577