Keeping timers relative to start of tunnel initiation is confusing

Keeping timers this way means that for example `update_session_timers` takes a `time_now` param that's a `Duration` instead of an `Instant`. All entries in `Tunn::timers` are `Duration`s when many of them are "the time that something happened" and idiomatically I think these would be better expressed as `Instant`s.