Add session loop.

Similar to the event loop and temporal loop, I'm adding a session loop to round everything off. Here's an example:

```
DECLARE session_index AS INTEGER
FOR EACH SESSION DELIMITED BY 2 HOURS
  SET session_index = session_index + 1
  FOR EACH EVENT
    SELECT count() AS count GROUP BY session_index
  END
END

# => {"1":{"count":10},"2":{"count":8}...}
```

I also removed the `sessionIdleTime` that you can pass into the JSON API. This is a more flexible alternative. 

/cc: @orenmazor @boourns @snormore