reduce
fun <K, V> KGroupedStream<K, V>.reduce( materialized: Materialized<K, V, KeyValueStore<Bytes, ByteArray>>, reducer: Reducer<V>): KTable<K, V>
fun <K, V> KGroupedStream<K, V>.reduce( name: String, materialized: Materialized<K, V, KeyValueStore<Bytes, ByteArray>>, reducer: Reducer<V>): KTable<K, V>