`None` is not a valid event id, so queuing up a database fetch for it seems
like a silly thing to do.
I considered making `get_event` return `None` if `event_id is None`, but then
its interaction with `allow_none` seemed uninituitive, and strong typing ftw.
We have to do this by re-inserting a background update and recreating
tables, as the tables only get created during a background update and
will later be deleted.
We also make sure that we remove any entries that should have been
removed but weren't due to a race that has been fixed in a previous
commit.
