private static byte[] value(String prefix, int i) {
    return toBytes(prefix, i);
  }
If this is called multiple times with the same {@code prefix} and any {@code i}, then this needs to return a byte[]
of the same length for each call.
samza-kv-inmemory/src/test/java/org/apache/samza/storage/kv/inmemory/TestInMemoryKeyValueStore.java