Added ServerStats struct.
Tracking stats of # accepted, closed, open conns.
REST /_api/stats URL.
Placeholder web U/I for server stats.
Web UI currently expects 1 sec stats intervals.

This should be calculated based on the server's stat-agg-freq but for
now just match it all to 1 sec for easier demo-ability.
Web U/I for server stats, and REST api.

Following/reusing the stats aggregation / passivation approach from
the existing bucket stats features.