[streaming] Added cosine distance and Euclidean distance as examples for possible delta function implementations. Test cases are included as well.

[streaming] Added policy helper for delta policies to provide a simpler API for such policies.

[streaming] Created ActiveEvictionPolicyWrapper. This eviction policy wrappes around a non-active policy and makes it active by forwarding notification on fake elements to the regular notification method.
[streaming] Integrated the policy based windowing into DataStream and introduced WindowedDataStream to handle windowing helper.
[streaming] Added test cases for the policy based windowing invokable.
[streaming] Created several examples for the usage of policy based windowing.

[streaming] Fixed windowing examples by removing manual setting of source parallelism

[streaming] Extended streaming guide with a section about the new policy based windowing.
[streaming] Introduced grouped windowing invokable, added required cloneable policy interfaces, and adjusted existing windowing invokable to make it usable together with the new grouped windowing invokable.
[streaming] Make all predefined policies cloneable and introduce an activating wrapper for cloneable eviction policies
[streaming] Adding unit tests for the grouped windowing invokable.
[streaming] New windowing API merge and cleanup + several minor fixes
[streaming] Extractor requirement removed for custom timestamp windowing
[streaming] Windowing API update + package refactor
[streaming] Window join api rework to support functional style
[streaming] Changed time based windowing policies to make lower boundary excluded and upper boundary included in the windows and adjusted policy test cases respectively.
[streaming] Windowing helper API cleanup
[streaming] Window cross API rework
[FLINK-1279] [streaming] Forward partitioning changed to use round-robin method
[streaming] Renamed test cases for window and grouped window invokable to be consistent with the invokable names
[streaming] Introduced central eviction policies in the grouped window invokable

[streaming] Extended test cases for grouped window invokable to cover usage of central eviction policies
[streaming] Central trigger/eviction API integration + invokable refactor
[streaming] Updated javadocs for new windowing semantics

[streaming] Updated streaming-guide to match recent api changes
[streaming] Changed GroupedWindowInvokable to always delete groups in case they have an empty element buffer