Add support for upper and lower bounds on datetimes

[FishmanL]

Specifically, so I can do things like
select avg(end_date-start_date) from table

[joshua-oss]

This is possible. For scenarios like the above, though, adding a column like "duration" can often have better utility, since you can bound the sensitivity more tightly. For example, the view exposed to SmartNoise could have a column that is computed as end_date - start_date, and the bounds could be set by the data curator to be reasonably tight based on domain expertise or some previous analysis on similar data that can provide a reasonable (e.g.) 1.5X IQR to remove outliers.

[FishmanL]

Yep, was planning to first-pass it with duration -- just trying to allow for more complicated things with datetimes than I might think of in advance (overall product is a wrapper around arbitrary queries)