Function derive_timer_event 

fn derive_timer_event(
    session_deadline: &mut Option<Instant>,
    error_deadline: &mut Option<Instant>,
) -> Event

Translate a timer expiration into the corresponding internal event.

Inspects which deadline is in the past (relative to now) and clears it, returning the matching Event. If both are simultaneously in the past, the session timeout wins (it is the more semantically meaningful of the two).