Add notify_onelevel.tdb

Add notify_onelevel.tdb

This optimizes non-recursive notifys. For non-recursive notifies we can use a
per-directory file-id indexed notify record. This matters for the Windows
Explorer and IIS cases which do not use recursive notifies. In these cases, we
do not have to shuffle around the whole notify record on every change.

For the cluster case, this improves correctness of the notifies, ctdb only
distributes the tdb seqnum once a second, so we can lose notifies.