[streaming] Serialization robustness improvements

[mbalassi]

Resolves the following issues:

• Streaming sometimes used Java's SerializationUtils instead of Flink's InstantiationUtils.
• Streaming stored the userfunctions twice in the operators, once named as userfunction, once named and typed specifically (e.g. flatMapper). With a tricky serialization we could end up with two distinct userfunctions after deserialization.

[gyfora]

It's a shame that we need to remove additional references to make the serialization work properly, but otherwise it looks good.

Let's merge it soon