[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!