Allow Partitioner to return -1 to indicate default partitioning

Prior to KIP-794 it was possible to create a custom Partitioner that could delegate to the DefaultPartitioner.  DefaultPartitioner has been deprecated so we can now only delegate to BuiltInPartitioner.partitionForKey which does not handle a non-keyed message.  Hence there is now no mechanism for a custom Partitioner to fallback to default partitioning, e.g. for the non-keyed sticky case.

I would like to propose that KafkaProducer.partition(...) not throw IllegalArgumentException if the Partitioner returns RecordMetadata.UNKNOWN_PARTITION and instead continue with the default behaviour.  Maybe with a configuration flag to enable this behaviour so as not to break existing expectations?

Why was Partitioner delegation with default fallback useful?

1. A single Producer can be used to write to multiple Topics where each Topic may have different partitioning requirements.  The Producer can only have a single Partitioner so the Partitioner needs to be able to switch behaviour based on the Topic, including the need to fallback to default behaviour if a given Topic does not have a custom requirement.
2. Multiple services may need to produce to the same Topic and these services may be authored by different teams.  A single custom Partitioner that encapsulates all Topic specific partitioning logic can be used by all teams at all times for all Topics ensuring that mistakes are not made.