Refactor: Take `their_node_id` by value across all handler interfaces

Refactor: Take `their_node_id` by value across all handler interfaces

In order to maintain interface consistency, we refactor all message
handler interfaces to take `PublicKey` rather than `&PublicKey`, as the
difference in efficiency should be negigible and the former is easier to
handle in binding languages.

Over time, we also want to move (no pun intended) towards all messaging
interfaces using move semantics, so dropping the reference for
`PublicKey` is the first step in this direction.