Use sensor_id

Requires migration described in: ``` cql/20141105_sensor_id.cql ```
I'd be much happier with a lot more testing around what sensors are marked for deletion and which ones will be created. At the moment, the creation and deletetion of the sensors are coupled with the insertion/deletion in c* so it is difficult to test. I think most the stuff in the ```let [...]``` blocks could be turned into separate functions that only take and return data and could be tested.