[session] make hook_control API private The doc already says "internal API". The old entry point is now officially deprecated and the context manager have been made private.

[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.