cubicweb #2551659 ldap: use entryUUID as extid instead of the dn [open]

We currently use the DN of an LDAP entity, which is not stable since it's a path to the object in an LDAP tree.

We should use its entryUUID as extid instead.

See the rfc4530 for details on this oprtational attribute.

done in<not specified>
closed by<not specified>