Expose a static Session closed event

[blakebyrnes]

We should have a static session closed event on Session (see new event) that broadcasts AFTER the database is fully shut down - this is among the last lines of Session.close(). It should include the id of the session, and probably the databasePath.

We should include an example that shows subscribing to this event and deleting databases. Possibly also loading the db using the SessionDb class (assuming someone wants to read internals).