Implement rd_kafka_conf_set_log_cb

[harmathy]

The default behavior of librdkafka is to log events like connection errors to stderr. In order to have the ability to change this behavior and set a custom logging function Kafka::Librd should implement rd_kafka_conf_set_log_cb.

[plicease]

Seems reasonable.

The config object gets created in the constructor. We should be able to pass in an optional CV to the constructor which can then be called via a C wrapper that calls the CV. The one tricky part is that the refcount on the CV will need to be incremented and decremented at the appropriate time. The CV can be stored in the rdkafka_t struct.

I don't have the time to work on this atm, but I would happily accept a PR and review.