implement "firstSeen" and "lastSeen" cluster properties

There are really two issues:
- we're not doing anything useful with the existing "firstSeen" and "lastSeen" properties (the properties are inheriting their zero value, documented here: https://golang.org/pkg/time/#Time)
- we're polluting the `data` properties in the cluster db record

A proposal: get rid of these two properties in the `Cluster` types we're using to submit to the database (or use another type definition); and then add these metadata in a meaningful way using a different type struct.