Add support for session window

[Micah Wylde]

Unlike hopping or tumbling windows which divide time up in fixed slices, session windows divide up time according to a minimum gap size.

Session windows will be introduced via a SQL function like the existing hop and tumble windows:

SELECT count(*), user_id
FROM pageviews
GROUP BY user_id, session(interval '10 minutes')

This is somewhat complicated by the fact that currently our SQL takes some shortcuts with handling window outputs that assume windows are fixed-length.

[Micah Wylde]

Closed via #243