`KafkaProducer`: refactor flushing

[felixschlegel]

Motivation:

With our own flushing implementation we were flushing until rd_kakfa_outq_len reached 0. However rd_kakfa_outq_len also takes other events such as statistics into account which led to a race where we were flushing for a very long time because more and more other events were produced to the queue while we were flushing.

Modifications:

• KafkaClient:
  • remove var outgoingQueueSize
  • add new method flush(timeoutMilliseconds:) that executes the blocking rd_kafka_flush call on a DispatchQueue but vends this as an async func
• KafkaProducer:
  • rename KafkaProducer.StateMachine.State.flushing to KafkaProducer.StateMachine.State.finishing
  • invoke KafkaClient.flush before terminating poll loop

[felixschlegel]

@FranzBusch : how shall we go about vending the flush timeout to the user?

[FranzBusch]

how shall we go about vending the flush timeout to the user?

We should put it into our configuration struct.