fix: prevent AsyncProducer retryBatch from leaking
fix: prevent AsyncProducer retryBatch from leaking

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

DataDog/sarama,Shopify/sarama,DataDog/sarama,Shopify/sarama