Storage for unhandled messages

[zviad-gvilava]

I have been using your library and find it to be quite beneficial for my projects. However, I've noticed that a key feature available in the MQTT.js library could enhance the usability and functionality of your library as well.

Specifically, I am referring to the "Store" feature described in the MQTT.js GitHub repository, which can be found at the following URL: [https://github.com/mqttjs/MQTT.js#store].