We model the getting of amounts as a network behaviour even though conceptually
it is a protocol. Refine/refactor the code a bit to make this more obvious.
- Use `Amounts` instead of `Messenger`
We only ever connect to a single peer, update peer tracker to reflect this. This
is a single patch because the handling of the two network behaviours is a
intertwined.
- Only track one peer connection
- Track the peer id and the multiaddr of the counterparty
- Emit an event for connection established on Alice's side as well as Bob's side
