update feature

````
   cardboard.update(primary, feature, dataset, callback)
````

 - [x] get current version of the feature, compute cover.
 - [x] compute cover for new version of the feature.
 - [x] delete any unused item (if the feature got smaller or moved)
 - [x] add / update the geometries in the other items or on s3 as needed
 - [x] if the feature is grow over 10k, then delete from dynamo item
 - [ ] preflight check
 - [ ] tests for change in index cover