Expose an endpoint to return estimate of total unique keys in cid.contact

[masih]

Pebble exposes key count estimation, which is exposed via Pebble store implementation API.

Indexstar already knows about all of dhstore instances, configured as --backend.

Expose an endpoint in indestar /stats to return the total number of unique pebble keys. Make sure to cache the requests to pebble, since getting the estimated count is expensive. So indexstar should cache the response of /stats and update as needed with expiry of 30 minutes or so.