[chat] Updates to chat functions

[seland]

Feature / Issue

Since we are adding multiuser chat, our interaction with Panthalassa will be changed. We need to make sure to check and update the following:

• [ ] how we get the list of chat
• [ ] how we send a message to the chat
• [ ] how we get messages
• [ ] how we mark messages as read
• [ ] what we pass to DApp as a context (identity key of partner)

Acceptance criteria

• [ ] All from the list above is tested and working