Make producers take a topic argument at send rather than init time

[rdiomar]

This allows a single producer to be used to send to multiple topics. See #110

Note: This is not backwards compatible

[dpkp]

Instead of breaking backwards compatibility, why not keep topic as an optional param in Producer inits and use that in message calls as a fallback if no topic is specified?

[rdiomar]

Since send_messages() takes a variable number of messages, it doesn't play well with optional params. If topic=None was just before *msgs, and we don't specify a topic when we call it, it will just get the value of the first message.

[dpkp]

I'm not a huge fan of the variadic *msgs interface, but you're right that we can't have topic be optional and maintain *msgs . Perhaps for future work it is worth considering something closer to the KeyedMessage interface used by the scala client.

Merging. This is not backwards compatible, but requiring a separate Producer instance for each topic is not scalable as discussed in issue #110.