Implement some basic aggregated metrics on listed origins

Metrics are computed and cached database-side by the update_metrics
function. The get_metrics function only retrieves the cached data.

The metrics are aggregated for each lister instance and visit type
(allowing complete reaggregation by visit type for cross-cutting statistics).

The following metrics have been implemented:

• number of known origins overall
• number of enabled origins (origins seen in the last listing)
• number of enabled origins that have never been successfully visited
• number of enabled origins with known activity since our last successful visit