Closes #1685

This closes #1744

This closes #1750

The fold API call takes an initial value as well as a fold function. In Scala it is possible
to provide an anonymous function. In order to easily support multi line anonymous functions
as well as being consistent with Scala's collection API, this PR adds another parameter list
to the fold API call, which contains the fold function parameter.

Insert spaces between first parameter list and curly braces of anonymous function

This closes #1748.

This closes #1751.

This closes #1749.

This closes #1743

This closes #1737

This brings it more in line with *ProcessingTimeWindows and makes it
clear what type of window assigner it is.

The old name, i.e. SlidingTimeWindows and TumblingTimeWindows is still
available but deprecated.

This enforces that the user always has to specify keys for both inputs
before .window() can be called.

This closes #1732.

This closes #1735.

The current flink-gelly-examples artifact id wrongly used an underscore to separate
examples from flink-gelly. This commit replaces the underscore with an hyphen.

This closes #1731.

This closes #1716.

This closes #1700.

Closes #1730

This implicitly adds KeyedStream.transform() and also explicitly
ConnectedStreams.transform().

This also removes the transform exclusions from the API completeness
tests.

This makes AllWindowedStream.fold() take constant space, just like the
keyed WindowOperator.

Also this adds a new test case in EventTimeAllWindowCheckpointingITCase
to verify that the FoldingWindowBuffer works.

This also renames the preexisting window buffers to ReducingWindowBuffer
and ListWindowBuffer to make the naming scheme consistent.

This closes #1723

This closes #1727

This closes #1725

The new flink-gelly-examples module contains all Java and Scala Gelly examples. The module
contains compile scope dependencies on flink-java, flink-scala and flink-clients so that
the examples can be conveniently run from within the IDE.

This closes #1719

[FLINK-3522] [storm compat] PrintSampleStream prints a proper message when involked without arguments

This also includes some minor cleanups

This closes #1689