Disentangle local user logic from networking logic

[SadieCat]

Description

Currently the logic for local users (LocalUser, etc) is very tangled up with the networking code (UserIOHandler, etc) and we should disentangle this.

Why this would be useful

This would allow us to implement things that don't involve users having a single socket e.g. a bouncer, or better session resumption, or gateway to another protocol that doesn't use persistent connections.

[SadieCat]

A good place to start with this would be to split the incoming/outgoing buffers into their own class from the networking code.