Streaming Windowing rework (runtime + api)

[Gyula Fóra-2]

Hey,

I have opened a PR for the windowing rework I have been doing in the last
couple of weeks.
It introduces several new abstractions for the windowing semantics along
with optional global/local discretisation logics and greatly improved
performance using pre-aggregators

https://github.com/apache/flink/pull/395

I will do some more testing and if I get some thumbs up I would like to
merge it soon. This should be a strict improvement over the current logic
both in terms of simplicity and performance. (with even more room for easy
improvement)

Cheers,
Gyula