[dbapi] makes anonymous_connection a computed property The current implementation is a boolean flag set manually by client code after connection creation. This led to different way to decide a anonymous_connection should be True (eg. different in the test than in the actual application code). It should not be client responsibility to set this flag. ``cnx.anonymous_connection`` is now a purely computed property. Connection with user in the "guests" group are anonymous, the other ain't. ``Session.anonymous_session`` is computed from ``cnx.anonymous_connection`` and get the updated behavior transparently. Closes #2953943

[webrequest] simplify set_session code Thanks to the previous changeset we are assured that session handed to set_session is full featured one. This allows a simpler code for this method. related to #2503918

[webrequest] set DBAPISession without cnx at initialisation time Such session are necessary for minimal use of a Request. Setting on by default allow simplification later linking with a full featured DBAPISession or equivalent. This was not possible before as all session was tracked by session manager. They are not tracked anymore since aa709bc6b6c1 and we can safely create them by default. related to #2503918