Maps a web interface member variable to a session field.
structSessionVar(T, string name);
Setting a SessionVar variable will implicitly start a session, if none
has been started yet. The content of the variable will be stored in
the session store and is automatically serialized and deserialized.
Note that variables of type SessionVar must only be used from within
handler functions of a class that was registered using
registerWebInterface. Also note that two different session
variables with the same name parameter will access the same