Include non-permanently connected peers in `list_peers`

[tnull]

As of #56, we allow to retrieve all permanently stored peers and add a is_connected: bool field based on their connection status.

It would however be nice to eventually return the superset of stored and connected (i.e., also peer not marked as permanently) peers.

This is much easier after lightningdevkit/rust-lightning#2056, which is why we postpone until then.

[tnull]

Blocked on #85 now.