Add mutexes around websockets

We replace the bare websocket.Conn type with a new Connection type which
implements `WriteJSON`, `WriteMessage`, and `Close`.  The Connection
type adds mutexes around writes since gorilla doesn't support concurrent
writes to websockets.

Signed-off-by: Honza Pokorny <honza@pokorny.ca>

connection.go

@@ -0,0 +1,33 @@
+package nostr
+
+import (
+	"github.com/gorilla/websocket"
+	"sync"
+)
+
+type Connection struct {
+	socket *websocket.Conn
+	mutex  sync.Mutex
+}
+
+func NewConnection(socket *websocket.Conn) *Connection {
+	return &Connection{
+		socket: socket,
+	}
+}
+
+func (c *Connection) WriteJSON(v interface{}) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	return c.socket.WriteJSON(v)
+}
+
+func (c *Connection) WriteMessage(messageType int, data []byte) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	return c.socket.WriteMessage(messageType, data)
+}
+
+func (c *Connection) Close() error {
+	return c.socket.Close()
+}