[FLINK-1325] [streaming] Added clousure cleaning to streaming

Streaming update for @aljoscha's recent [PR](https://github.com/apache/incubator-flink/pull/269).

The functionality is there, the code could be a bit nicer but as @gyfora has already refactored the builder methods in the DataStream for the streaming [scala-api](https://github.com/mbalassi/incubator-flink/tree/scala-api) and not yet merged it this solution will be more straightforward to merge.

As this is part of the recently merged PR @aljoscha and myself are personally for adding it to the 0.8 release (with this codebase, not the merged one). 
We don't need the extra cleaning before the .getClass() method. This also applied to the other parts in this PR where that pattern occurs.