Record a 'last seen' timestamp for hosts in the host store

We should record a timestamp of the last time we saw information about a host in the host store.

Applications could use this timestamp to determine if the information is sufficiently recent that they can use it, or they can disregard information that is too old.

Ideally we could record this timestamp per IP address on a host - if this is too tricky we can simply record last seen time for the entire host.