Add fiber implementation for Kafka producer client

[Tim-Brooks]

I was playing around with Comsat and Quasar this weekend to checkout the current state of development.

I wanted to to try an implementation, and created (what I think) is a working implementation of the Kafka producer. I started on a consumer implementation, but the current release Kafka "newer" consumer is unstable and the interface will change noticeably in the next release. So I thought I would hold off on that.

I thought I would submit this pull request in case this functionality of interest.

[pron]

Hi, and thank you very much!
Am I to understand correctly that the only change is returning a Future that can be joined by a fiber?

[Tim-Brooks]

Yeah. The Kafka Producer is already asynchronous with an io thread flushing the messages in the background. The future is completed whenever the send completes with a configurable number of acks.

It seemed to me, based on other examples, that making it compatible with Quasar only required integrated with the SettableFuture.

[pron]

Yep. Looks good.
Could you perhaps add a test that actually joins the future in a fiber?

[Tim-Brooks]

Yeah. I'll rework the tests a bit later to test that.

[Tim-Brooks]

In the most recent commit, I modified the tests so that the futures were being joined inside a fiber.

[pron]

Very good. Thank you very much!