callback invocation for docs added during couch_changes startup can be delayed by race condition

After subscribing to notifications it's necessary to re-open the #db a so that the header points at all updates for which the updater notifier has already fired events. In practice, this is rarely problematic because the next change will cause everything to catch up, but if a quick burst of changes happens while, e.g., replication is starting the replication can go stale. Detected by intermittent replicator_db js test failures.