range balancer: use correct members for div/rem

Issue redpanda-data#24 showed that the range balancer was broken. On first glance,
the issue appears to be because the consumerIdx wraps past the end of
potentialConsumers, and the obvious fix is to just inc and mod. This
would be an incorrect fix.

The real problem lies above in determining div and rem. The whole point
is that (div*(potentialConsumers) + rem) == num partitions, thus if we
drain div and one from rem every loop, then we will never increment past
the end of potential consumers, because partitions will be completely
drained on the last consumer.

The prior logic accidentally made div smaller and rem larger, thus
giving too few partitions to each consumer, and then in some scenarios,
would cause us to exhaust consumers while still draining partitions.

I will be adding unit tests later.

pkg/kgo/group_balancer.go

@@ -293,7 +293,7 @@ func (*rangeBalancer) balance(members []groupMember, topics map[string]int32) ba
 			partitions[i] = int32(i)
 		}
 		numParts := len(partitions)
-		div, rem := numParts/len(members), numParts%len(members)
+		div, rem := numParts/len(potentialConsumers), numParts%len(potentialConsumers)
 
 		var consumerIdx int
 		for len(partitions) > 0 {