producer: add kafka_msg to returned futures
producer: add kafka_msg to returned futures

Users will likely always want to have the message content stapled
together with the delivery-future, for context when delivery fails, so
doing that for them makes things that bit more convenient.

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

thedrow/samsa,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,wikimedia/operations-debs-python-pykafka,thedrow/samsa,benauthor/pykafka,benauthor/pykafka,wikimedia/operations-debs-python-pykafka