Librdkafka upgrade

Issue

We have some issues seen in the Kafka communication. These seem to have existed for other users of librdkafka.

Description of work

There are some clear issues which look like they could be fixed simply by an upgrade of librdkafka. This PR implements that, and updates a mock producer so it compiles successfully on the external brokers. A comment was added to the MockMessage helper making this explicit for any future users.