[service] split session retrieval and service execution Internal session are not tracker by the repository and can not be retrieved using _getsession. Moreover their session id is not unique… Closes #2735700

[service] handle cnxset in the async case only The service may be called by a session, we really do not want the call to service to mess with the transaction state. This does not apply to async were thread magic ensure a dedicated transaction.

[service] extract session retrieval from "task" closure The session was retrieved there because `set_cnxset` must be called in the same thread as the code using it. We now retrieve the session beforehand and explicitly call `set_cnxset`.