fix: Don't allow peer_disconnect before peer_connected callbacks

The previous implementation would send a peer_disconnect callback for
any error after the NOISE handshake completed, but only send a
peer_connected callback after the Init message was received. This could
lead to a dangling peer_disconnected callback that didn't have a paired
peer_connected.

This patch cleans up the state differentiation to match the following
cases:

unconnected (!transport.is_connected() && peer.post_init_info.is_none())
* Know about the peer, but are still in the process of the NOISE
handshake

connected (transport.is_connected() && peer.post_init_info.is_none())
* NOISE handshake completed, but haven't received an Init message

initialized (transport.is_connected() && peer.post_init_info.is_some())
* The NOISE handshake has completed and the Init message has been
received and processed

With the 3 conceptual states, the read_event() path now only inserts
a Peer into the node_id_to_descriptor map after a Peer enters the
initialized state.

This fixes the asymmetry between peer_disconnected & peer_connected as
well as simplifies the disconnect code.

Author: julianknutsen