Archiver needs to handle database restarts

The archiver lost its connection to the database, due to a (security?) update made to the database server. I don't think it ever tried to reconnect, but kept caching new rows after the failure in the attached log segment.

I don't think there is a trivial way to restart, but the hint at https://github.com/psycopg/psycopg2/issues/419 is probably good enough to implement from.