[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.