Revert "BLESession.notify: Send out notifications at once to avoid interruption"

This reverts commit 2c7a10f848.
It turned out that string join with '\n' is not the proper way to pack
multiple JSON messages to Scratch Lego Boost extension.

scratch_link.py

@@ -95,22 +95,20 @@ class Session():
         Notify BT/BLE device events to scratch.
         """
         logger.debug("start to notify")
-        # merge all notifications queued
-        notifications = []
+        # flush notification queue
         while not self.notification_queue.empty():
             method, params = self.notification_queue.get()
-            notifications.append(self._build_notification(method, params))
-        # send merged notifications
-        future = asyncio.run_coroutine_threadsafe(
-            self.websocket.send('\n'.join(notifications)), self.loop)
-        result = future.result()
+            self._send_notification(method, params)
 
-    def _build_notification(self, method, params):
+    def _send_notification(self, method, params):
         jsonn = { 'jsonrpc': "2.0", 'method': method }
         jsonn['params'] = params
         notification = json.dumps(jsonn)
         logger.debug(f"notification: {notification}")
-        return notification
+
+        future = asyncio.run_coroutine_threadsafe(
+            self.websocket.send(notification), self.loop)
+        result = future.result()
 
     async def handle(self):
         logger.debug("start session hanlder")