SMSUsers API and Checkpoints

It feels strange to me that checkpoints would be maintained in ilsgateway instead of commtrack. what's the thought here? 

typically what we've found most successful is being able to query by date in the API, and then storing the checkpoint date on the caller's side.

also if you do things based on incrementing primary keys then it won't work with any edits to the data. so i think we should first try and make sure we can do as much as possible using "last_modified" dates whereever possible, and then if there are models that don't have data we can use figure out something else to do with those