s4:dsdb: add dsdb_notification module

s4:dsdb: add dsdb_notification module

This adds a simple implementation of LDB_CONTROL_NOTIFICATION_OID.
It requires caller (the ldap server task) to retry the request peridically,
using the same ldb_control structure in order to get some progress and
the never ending search behaviour an LDAP client expects.

For now we remember the known_usn in a cookie stored
in the otherwise unused ldb_control->data fielf
and we do a simple search using (uSNChanged>=${known_usn}+1).

In future we may do things based on the uSNChanged value.

for (i = old_highest + 1; i <= current_highest; i) {
	search for (uSNChanged=i)
}

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>