Call `raven.update_coinc_far` and potentially `raven.trigger_raven_alert` serially

We seen issues with https://gracedb.ligo.org/superevents/S231102av/view/ where a hung raven.update_coinc_far changed the preferred external event info to before there was sky map info (not the correct behavior). This could have been fixed by calling this function by the exttrig worker which has a concurrency of 1.

It could also make sense to call the parent function raven.trigger_alert_alert to only have one event trigger at a time and make the message calls more orderly.