Refactor Channel to more clearly separate "single session per user" from "multiple session per user"

[snopoke]

Slack and Web channels are 'multi-session' per user where each user / participant can have multiple active sessions. Other channels like Telegram, Whatsapp etc can only have a single active session.

Consider splitting the ChannelBase class to more clearly delineate the differences.

Also refactor test_base_channel_behaviour. Currently we use the telegram channel as a vehicle to test this, but ideally we should be using all channel types.