Check for MongoDB Replica Set state on startup

When the replica set is in recovery state, sometimes migrations hang because the updates in MongoDB don't finish. This is hard to spot from the startup logs (there is no message telling of this) but only because the migrations don't finish and we never see the "has started" message.

Maybe we should issue a warning on startup or find a way to exit with an error if there is a migration supposed to run that modifies MongoDB data? It is probably not an issue in most cases (although you don't want that state for the Replica Set) if it's just a regular startup of Graylog.

e.g. see https://community.graylog.org/t/cant-connect-to-web-interface-after-upgrade-to-6-0/36673/11