Fix bug where we don't get new to-device from remote if they resent a
message we've already persisted and have recorded in the DB twice.
`device_federation_inbox` table doesn't have a unique index, and so we
can race and store an entry in there twice. If we do so then
`simple_select_one_txn` will throw an error due to the query returning
more than one row. We should add an unique index, but it doesn't really
matter so lets just handle the case of multiple rows correctly for now.
