implement "firstSeen" and "lastSeen" cluster properties

[jackfrancis]

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.