fix: ensure backoff timer is re-used

Unlikely to cause an issue in Sarama unless the user explicitly set
Metadata.Retry.Backoff especially high and was hitting lots of retry
conditions, but no harm in guarding against this.

Ref: https://medium.com/@oboturov/golang-time-after-is-not-garbage-collected-4cbc94740082

consumer_group.go

@@ -732,6 +732,9 @@ func (s *consumerGroupSession) heartbeatLoop() {
 	pause := time.NewTicker(s.parent.config.Consumer.Group.Heartbeat.Interval)
 	defer pause.Stop()
 
+	retryBackoff := time.NewTimer(s.parent.config.Metadata.Retry.Backoff)
+	defer retryBackoff.Stop()
+
 	retries := s.parent.config.Metadata.Retry.Max
 	for {
 		coordinator, err := s.parent.client.Coordinator(s.parent.groupID)
@@ -740,11 +743,11 @@ func (s *consumerGroupSession) heartbeatLoop() {
 				s.parent.handleError(err, "", -1)
 				return
 			}
-
+			retryBackoff.Reset(s.parent.config.Metadata.Retry.Backoff)
 			select {
 			case <-s.hbDying:
 				return
-			case <-time.After(s.parent.config.Metadata.Retry.Backoff):
+			case <-retryBackoff.C:
 				retries--
 			}
 			continue