api: ensure mqtt futures exists as attributes

We were running into a bug where, during a server
initiated disconnect, the client would try to read
an attribute on its instance that had never been written
to. This resulted in an AttributeError exception being
thrown that effectively killed the whole MQTT task
and kept the app from ever doing any MQTT related things
again.

api/src/dooris_api/mqtt_client.py

@@ -78,6 +78,10 @@ class AsyncMqttClient:
         self.client.on_subscribe = self.on_subscribe
         self.client.on_unsubscribe = self.on_unsubscribe
         self.client.on_disconnect = self.on_disconnect
+        self.fut_disconnect = None
+        self.fut_connected = None
+        self.fut_subscribe = None
+        self.fut_unsubscribe = None
 
     def on_connect(
         self,