Add storage backend to chat server

[banool]

Currently messages are lost if the server restarts since it's all just in memory. Add some kind of storage backend. Most reliable would be some kind of write-through cache, though you could also just have some kind of periodic flushing to storage.

The solution here needs to take into account:

• Encrypted messages at rest.
• The possibility of using the Signal protocol or something like it.
• Having multiple chat server instances.
• Sharding storage.
• etc.

All of these can come later though and should comprise their own issues.