[FLINK-1147][Java API] TypeInference on POJOs

[twalthr]

The TypeExtractor now also fully supports generic POJOs and tries to get missing types by using type inference.

Functions can look like:

MapFunction<PojoWithGenerics<Long, T>, PojoWithGenerics<T,T>>
MapFunction<Tuple2<E, D>, PojoTuple<E, D, D>>
MapFunction<PojoTuple<E, D, D>, Tuple2<E, D>>

POJOs can contain fields such as:

public PojoWithGenerics<Z, Z> field;
public Z[] field;
public Tuple1<Z>[] field;

If you have ideas for other test cases, I'm happy to implement them.

[StephanEwen]

Great! I am looking forward very much to having that in the APIs

[rmetzger]

I didn't find any issues in the code. Good to merge.

[StephanEwen]

+1 to merge from me as well. Go ahead, Timo...