Fix broker asyncness

You can actually have a second thread send while the first is still waiting for
a response now. The previous changes had the sender hold the lock the entire
time, which was silly. That's what the channel is for.

broker.go

@@ -99,31 +99,45 @@ func (b *Broker) Produce(clientID *string, request *ProduceRequest) (*ProduceRes
 	return response, nil
 }
 
-func (b *Broker) sendAndReceive(clientID *string, req requestEncoder, res decoder) error {
+func (b *Broker) send(clientID *string, req requestEncoder, promiseResponse bool) (*responsePromise, error) {
 	b.lock.Lock()
 	defer b.lock.Unlock()
 
 	fullRequest := request{b.correlation_id, clientID, req}
 	buf, err := encode(&fullRequest)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	_, err = b.conn.Write(buf)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	b.correlation_id++
 
-	if res == nil {
-		return nil
+	if !promiseResponse {
+		return nil, nil
 	}
 
 	promise := responsePromise{fullRequest.correlation_id, make(chan []byte), make(chan error)}
 	b.responses <- promise
 
+	return &promise, nil
+}
+
+func (b *Broker) sendAndReceive(clientID *string, req requestEncoder, res decoder) error {
+	promise, err := b.send(clientID, req, res != nil)
+
+	if err != nil {
+		return err
+	}
+
+	if promise == nil {
+		return nil
+	}
+
 	select {
-	case buf = <-promise.packets:
+	case buf := <-promise.packets:
 		return decode(buf, res)
 	case err = <-promise.errors:
 		return err