MINOR: putting back kstream stateful transform methods

[ymatsuda]

@guozhangwang

• added back type safe stateful transform methods (kstream.transform() and kstream.transformValues())
• changed kstream.process() to void

[guozhangwang]

The functionality of process() now is completely covered by transform: users can define a transform function with return type R be "Void" and add a dummy "return null" in the end of the function. And then in KStream we can add

public void transform(TransformerSupplier<K, V, Void>)

to replace the "process()" call. Having both process() and transform() might be confusing to users, so I would suggest we just remove process() here.

[guozhangwang]

This would require a separate KStreamVoidTransformProcessor, but I feel it worth the internal cost for simpler public APIs.

[guozhangwang]

Talked to @ymatsuda offline about the semantics of process() and transform(), I am now convinced that we may better keep both of them since making transform() return void may be as awkward for end users as well. so LGTM.