The Java/Scala Kafka client supports a mechanism for sending
messages asynchronously by using a queue and a thread.
Messages are put on the queue and the worker thread keeps sending
it to the broker.

This ticket implements this feature in python
We use multiprocessing instead of threads to send the messages

Conflicts:
	kafka/producer.py

Add support for two options in the producer - req_acks and ack_timeout
The acks, if any, are passed to the caller directly

Also, ensure that the case of 'no-acks' works fine
In conn.send(), do not wait for the response. Wait for it only on
conn.recv(). This behaviour is fine now since the connection is not
shared among consumer threads etc.

Also improve on the logic for stopping the async Processor instance.
Ensure that unsend messages are sent before it is stopped.