Add statistics endpoint for monitoring & tracking server stats

[kriswehner]

Right now, we don't have any stats exposed for production monitoring AFAICT.

We need to know at least:

• Total requests served
• Total bytes transmitted
• Number of connected clients (this can easily be derived from exposed counters of requests started / requests finished)
• Number of errored clients
• Earliest and latest database LSN / record time
• Whether or not the replication connection is up, and number of replication connection flaps

This can be done as simply as using "expvar" perhaps.