Add a write_kafka output with similar properties to the amqp one.

This module leverages the librdkafka library to embed a kafka producer within collectd.
Value Lists are formatted as commands, graphite output or json.

The partition key assigned to payloads will either be random if no key is provided or will be based on the CRC32 of an arbitrary string.

In a setup where each host publishes its own events to a kafka broker, using the hostname or the host group as a key will allow sane partitioning.
What do riemann protobufs have to do here ? It looks like a mistake to me, but I might be wrong.