[client-connection] handle the lack of connection id while not open the connection id is computed at opening and forgotten when closing. We can't rely on it in various messages … like the "connection closed" exception.

[connection] transparent cnx_set handling Connection object while take cares of there cnxset themself (as dbapi connection does). The ``set_cnxset`` and ``free_cnxset`` operation are still available for backward compatibility purpose. The ``_auto_free_cnx_set`` is introduced to handle mixed usage. A new context manager ``connection.ensure_cnx_set`` is added for code that access ``cnx.cnxset`` directly and are not wrapped in any specific ``Connection`` method. A ``_with_cnx_set`` decorator is used on all Connection method that need a cnxset.