Do not overly protect writes

Follow-up to #161

cc @fadams

channel.go

@@ -237,8 +237,6 @@ func (ch *Channel) sendOpen(msg message) (err error) {
 			return ch.sendClosed(msg)
 		}
 
-		ch.connection.startSendUnflushed()
-
 		// Flush the buffer only after all the Frames that comprise the Message
 		// have been written to maximise benefits of using a buffered writer.
 		defer func() {

connection.go

@@ -448,15 +448,10 @@ func (c *Connection) send(f frame) error {
 	return err
 }
 
-// This method is intended to be used with sendUnflushed() to start a sequence
-// of sendUnflushed() calls
-func (c *Connection) startSendUnflushed() {
-	c.sendM.Lock()
-}
-
 // This method is intended to be used with sendUnflushed() to end a sequence
 // of sendUnflushed() calls and flush the connection
 func (c *Connection) endSendUnflushed() error {
+	c.sendM.Lock()
 	defer c.sendM.Unlock()
 	return c.flush()
 }
@@ -481,7 +476,9 @@ func (c *Connection) sendUnflushed(f frame) error {
 		return ErrClosed
 	}
 
+	c.sendM.Lock()
 	err := c.writer.WriteFrameNoFlush(f)
+	c.sendM.Unlock()
 
 	if err != nil {
 		// shutdown could be re-entrant from signaling notify chans