feat: introduce `Tunn::time_since_last_received`

[thomaseizinger]

In addition to time_since_last_handshake, expose a time_since_last_received. This is useful to know downstream because it allows applications to react to connectivity problems. Keepalive packets will update this timer. As soon as this duration is > than the configured keep-alive (of the remote), we know that there is a connectivity problem.