[client-connection] remove the _srv_cnx usage It does not do anything special now that we use explicite Connection object with automatic cnx_set handling.

[client-connection] explicitly check that the client-connection is open The check is also perform by the _srv_cnx property. But we do not need those property anymore.

[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.

[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.

[connection] invert __init__ parameter Takes session first. At some point, the connection_id will become optional for Connection created explicitly.

[service] enforce that Service argument and return are json-serialisable The call_service API need to be able to run through RPC. So we ensure front start that it is possible serialise both input and output.

[connection] move call_service on Connection This is the last step toward standalone transaction.

[connection] move the commit method on Connection object One step closer of standalone Connection!