[repoapi] add an anonymous_cnx function This function return an anonymous ClientConnection. It aims to replace ``dbapi.anonymous_session``.

[repoapi] add a connect function This function takes a repo and authentication information to open a new Session and return a ClientConnection associated to it.

[client-connection] add an auto-close property for ClientConnection The next commit introduce a connect function that open a new Session and return an associated ClientConnection. The Session should not be used by anything else than the created ClientConnection, so we want to close it at the same time than the ClientConnection. The implementation in this changeset is simplistic. We probably want to move this notion in the session itself. This be improved once we have server side Connection