set initial namespace to sessionID, to support multiple users

what do these session ids look like? I know in some contexts they are UUIDs, which would be rather long. Being able to feasibly type the namespace name is important I think. There's also the noise of the vars returned from defs: after `(def foo 42)` you could see a return value like

``` clojure
#'session-9cc76f32-4dd7-4827-839e-947f2ba5d168/foo
```

I think ideally we could do `session-N` with an auto-incrementing integer but that would require the server to generate the namespace to ensure uniqueness.