Fix race condition where preferred external event chosen incorrectly

There is a current issue with how the RAVEN pipeline chooses a preferred external event, either choosing the wrong external event or mixing info from different events. This is due to the available data from the IGWN alert being stale by the time this check is made, since the previous processes (search for coincidences, calculate joint FAR) can take several seconds to complete. Therefore, if several instances of this pipeline are running simultaneously (which often occurs if multiple GRB candidate), this stale info can lead to incorrect decisions.

An easy fix is to add a GraceDB poll that gets the latest superevent info prior to checking, which should allow simultaneous instances of the RAVEN pipeline to communicate.

An example of different events being mixed up (used external event ID and space_coinc_far from different events): https://gracedb-playground.ligo.org/superevents/MS230118f/view/