Get Rid of Cast from ProcessorContext to InternalProcessorContext

The following cast is often used in Kafka Streams code.

public void init(final ProcessorContext context) {
    internalProcessorContext = (InternalProcessorContext) context;
    ...
}

This code leads to a ClassCastException if the implementation of the ProcessorContext is not an InternalProcessorContext, which defeats the purpose of using interface ProcessorContext in the API.