refactor(mqtt): change keepalive to 30s

src/utils/mqtt.js

@@ -24,7 +24,7 @@ const options = {
   username,
   password,
   clientId: `mqttjs_${Math.random().toString(16).slice(2)}`,
-  keepalive: 60,
+  keepalive: 30,
   protocolId: 'MQTT',
   protocolVersion: 4,
   clean: true,