Extend / change API behavior for sending/receiving messages

[PHameete]

Referring to #10

If you create a consumer for a topic, and produce a message for that topic directly afterwards on the same thread, you will likely not receive the message on the consumer.

This is not the behavior you would expect, and it also makes unit testing difficult.

Two possible solutions come to my mind:

1. Make consumer creation synchronous by forcing it to fetch partition info before returning
2. Allow consumers to fetch by offset specifically, and let producers return the offset of a produced message.