Can't create a second session without restarting at-driver server.

[gnarf]

If you connect with a websocket and start a session, I'm noticing a few problems that don't get in the way of the automation service, but do get in the way of a developer working on the harness.

• The session is not closed or ended when websocket disconnects, and the server won't allow a second socket to create a new session resulting in:

[Agent]: [debug] [message raw] {"id":"0","error":"session not created","message":"session already exists"}

• The second websocket starts receiving messages for the previous websocket's session

In combination, they end up kind of working out as usable, but if I were to start relying on the response to the session.new the system would break.