[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
