[Feature request] Local persistence

Hi,

I find myself in a situation where I need a local persistence of the MQTT messages when my devices loose the internet connection and reboot. It works perfectly when the device just loses the connection and reconnects later on, but in the case of a hard reboot I loose every unsent messages, which is not good for my use cases. I would like to see this kind of feature in the native library and avoid developing it myself (which i'll probably have to do at some point)

If that's already possible somehow, i'd be glad to know how :)