Feature Request - Expiration on messages to allow for ephemeral messaging

[garthvh]

• [ ] Expire time for messages like waypoints
• [ ] Don't send ephemeral messages over MQTT
• [ ] Delete from device (if still on the device) if the timer has elapsed

[geeksville]

This issue has been mentioned on Meshtastic. There might be relevant details there:

https://meshtastic.discourse.group/t/disappearing-messages-like-signal/9297/2