multi-session support

[alex-berman]

Currently, the dialog state is shared across sessions, if I understand correctly. For example, when I currently open the demo in a new browser window and ask "what do you predict?", the system responds with "The instance with id equal to 535 is predicted unlikely to have diabetes.", presumably because I asked about this instance in another session.

It would be preferable if, instead, each session had its own dialog state.