fix connection close race

Need to prevent Close from being called at the same time. The lock
already existed, but was only used in one place where it should be
applied generally.

conn.go

@@ -31,6 +31,12 @@ func (c *conn) sendPacket(m encoding.BinaryMarshaler) error {
 	return sendPacket(c, m)
 }
 
+func (c *conn) Close() error {
+	c.Lock()
+	defer c.Unlock()
+	return c.WriteCloser.Close()
+}
+
 type clientConn struct {
 	conn
 	wg         sync.WaitGroup
@@ -67,9 +73,7 @@ func (c *clientConn) loop() {
 // appropriate channel.
 func (c *clientConn) recv() error {
 	defer func() {
-		c.conn.Lock()
 		c.conn.Close()
-		c.conn.Unlock()
 	}()
 	for {
 		typ, data, err := c.recvPacket()