Provide round-robin mode in SubPartitioner for non-null key

[ocadaruma]

• Currently, Decaton's SubPartitioner uses murmur2 hash of the key to determine the subpartition to ensure per-key processing ordering guarantee.
• However, sometimes we want to use round-robin strategy even for non-null key. e.g.:
  • Topic is consumed by multiple consumers, and some consumers need ordering guarantee (e.g. windowed-processing) but some consumers don't (just collecting stats).
  • In latter type of consumers, using hash-partitioning doesn't make sense. (just spoils the processing throughput when key-bursting happens)
• It's nice to have a feature to enable round-robin mode even for non-null keyed topics.