Fix issue with re-binding to an LDAP connection (T1318)

Summary:
Fixes T1318

Note that set_entry_attributes() will now use two LDAP connections, which may
or may not be what we want. I didn't find a simple way of using ldap_priv connection
in entry_dn() and get_entry_attributes().

Reviewers: PyKolab Developers

Maniphest Tasks: T1318

Differential Revision: https://git.kolab.org/D188