Stats

In an effort to get some insight into the state of replication (and to be able to graph the replication backlog), add --stats option to radosgw-agent.

--stats will cause radosgw-agent to run, output a large blob of JSON, and exit.  The JSON contains the shard data that radosgw-agent uses to do replication, as well as some some sums and max values from the individual shards.  If --stats --metadata-only is supplied, the JSON does not include data in the JSON output.

I tried to follow the existing coding style, but I'm not a python coder.  Please let me know if I did anything embarrassing.