count unique cases modified

I realized that the previous version wasn't going to work since it only kept track of the most recent modification to a case by a user. Hence 'cases updated in month 0' and 'cases updated in month 1' would likely be inconsistent.

This approach dumps ALL case modifications into SQL with the case_id and then does a `count(disticnt(case_id))` to get the number of cases modified in the date range.

Requires re-index :(