feat: more fine-granular timer update

[thomaseizinger]

At present, it is up to the host app how often it calls update_timers_at. Once a second should be enough but ideally, a Tunn just exposes when the next internal timer fires. This is being tackled more generally in #61. In order to ensure we send keep-alive messages in time, we incorporate want_passive_keepalive_at into the next_timer_update into it.