Fix PartitionConsumer race condition on Close.

consumer.go

@@ -295,7 +295,7 @@ func (c *Consumer) Close() (err error) {
 		for range c.errors {
 		}
 		for p := range c.partitions {
-			_ = p.Close()
+			p.AsyncClose()
 		}
 		for range c.notifications {
 		}

partitions.go

@@ -86,7 +86,7 @@ func (c *partitionConsumer) InitialOffset() int64 { return c.initialOffset }
 // AsyncClose implements PartitionConsumer
 func (c *partitionConsumer) AsyncClose() {
 	c.closeOnce.Do(func() {
-		c.closeErr = c.PartitionConsumer.Close()
+		c.PartitionConsumer.AsyncClose()
 		close(c.dying)
 	})
 }