fix: data race in roundrobin balancer

[petedannemann]

#1232 introduced a data race in the roundrobin balancer. It isn't a meaningful one, but still shows up when you use kafka-go as a library and run go test -race

[LKaemmerling]

We encountered this issue as well, would be great to get this merged soonish :)

[4ND3R50N]

I´m experiencing the same issue, our tests are failing when checking with -race

[LKaemmerling]

Quickfix (without this MR, but this MR is preferred):

- Set the Balancer to RoundRobin explicitly and set ChunkSize to at least 1

w := &kafka.Writer{
	Addr:     kafka.TCP("localhost:9092", "localhost:9093", "localhost:9094"),
	Topic:   "topic-A",
	Balancer: &kafka.RoundRobin{
			ChunkSize: 1,
		},
}

[petedannemann]

Fixed in https://github.com/segmentio/kafka-go/releases/tag/v0.4.47

[4ND3R50N]

That was fast! thank u guys!