Fix an additional possible alignment issue

Also add comments so future me knows why those variables are all by
themselves at the top of the structs.

consumer.go

@@ -289,10 +289,11 @@ type PartitionConsumer interface {
 }
 
 type partitionConsumer struct {
-	consumer  *consumer
-	conf      *Config
-	topic     string
-	partition int32
+	highWaterMarkOffset int64 // must be at the top of the struct because https://golang.org/pkg/sync/atomic/#pkg-note-BUG
+	consumer            *consumer
+	conf                *Config
+	topic               string
+	partition           int32
 
 	broker   *brokerConsumer
 	messages chan *ConsumerMessage
@@ -302,9 +303,8 @@ type partitionConsumer struct {
 	trigger, dying chan none
 	responseResult error
 
-	fetchSize           int32
-	offset              int64
-	highWaterMarkOffset int64
+	fetchSize int32
+	offset    int64
 }
 
 var errTimedOut = errors.New("timed out feeding messages to the user") // not user-facing

mocks/consumer.go

@@ -178,7 +178,7 @@ func (c *Consumer) ExpectConsumePartition(topic string, partition int32, offset
 // Errors and Messages channel, you should specify what values will be provided on these
 // channels using YieldMessage and YieldError.
 type PartitionConsumer struct {
-	highWaterMarkOffset     int64
+	highWaterMarkOffset     int64 // must be at the top of the struct because https://golang.org/pkg/sync/atomic/#pkg-note-BUG
 	l                       sync.Mutex
 	t                       ErrorReporter
 	topic                   string