Clean up stats

This pull request cleans up `Metrics` and adds several new timers and gauges. I verified that the stats are sent to statsd...do we want to migrate to Heka's `StatsdInput` at some point?

* Track message routing responses and latencies.
* Add gauges for tracking the number of connected clients.
* Capture timing information as soon as possible.

Closes #41.
I think this mutex can be removed—there's no guarantee statsd will receive the `Gauge` packet before `GaugeDelta` (if at all), and this won't help if we're using a single statsd server per cluster.