Hi @pnowojski ! 

I can understand that this can be an interesting addition for some usecases, 
but it is a big one, and it should be discussed more thoroughly and, most 
importantly, more publicly. I would be against merging it as just a sub-commit 
of another feature.

The reason is that this allows users to "play" with watermarks from the level 
of a `Function` and not `Operator`, which was, intentionally, the case so far.

If you want to "hold back" the watermark, the this should be done by a 
watermark assigner.

If you want to run a "callback" upon watermark, then so far the trick is to 
register a timer for `watermark + 1`. 

I can find usecases which do not fall into any of the above, but for those so 
far we implement custom operators. 

[ Full content available at: https://github.com/apache/flink/pull/6687 ]
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