bugfix: AsyncProducer retryBatch causes goroutines to be leaked

retryBatch caused the brokerRefs count to be incremented (getBrokerProducer), but it is never decremented again, so the
goroutines related to the brokerProducer are leaked.

async_producer.go

@@ -1064,6 +1064,7 @@ func (p *asyncProducer) retryBatch(topic string, partition int32, pSet *partitio
 	}
 	bp := p.getBrokerProducer(leader)
 	bp.output <- produceSet
+	p.unrefBrokerProducer(leader, bp)
 }
 
 func (bp *brokerProducer) handleError(sent *produceSet, err error) {