[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.

[client-connection] explicit the client part in __repr__ Now that we have real server side connection we need to remove ambiguity.

[ClientConnection] directly use the Connection object to access the database Now that Connection are a full featured standalone object we can directly reference and use it in the ClientConnection instead of using the session. The session object is kept around for a while to perform various utility role.