ldap: Make ldap2 connection management thread-safe again
    
    This fixes the connection code in LDAPClient to not store the LDAP connection
    in an attribute of the object, which in combination with ldap2's per-thread
    connections lead to race conditions resulting in connection failures. ldap2
    code was updated accordingly.
    
    https://fedorahosted.org/freeipa/ticket/5268
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>