[core] Protect session data from unwanted loading. Use specialised Session and Connection types that forward their 'data' and 'session_data' attributes to the pyramid request.session attribute. This forwarding is done with properties, instead of copying a reference, which allow to access request.session (and the session factory) if and only if Session.data or Connection.session_data is accessed. In some cases, most notably the static resources requests, it can mean no access the session during the request handling, which saves a request to the session persistence layer. Closes #4891437

pyramid