Memory leak in the confluent kafka producer

[yanmxa]

@embano1 I just found a memory leak in the confluent kafka protocol Sender. I apologize for overlooking this issue in the initial version.

The producer should fetch the event from the producer.Events() chan. Otherwise, the events chan will eventually fill up.

Reference: confluentinc/confluent-kafka-go#1043, confluentinc/confluent-kafka-go#578