stats: Add network and http statisics (#3686)

Network: total bytes of incoming and outgoing server's data
by taking advantage of our ConnMux Read/Write wrapping

HTTP: total number of different http verbs passed in http
requests and different status codes passed in http responses.
This is counted in a new http handler.

cmd/routers.go

@@ -84,6 +84,8 @@ func configureServerHandler(srvCmdConfig serverCmdConfig) (http.Handler, error)
 
 	// List of some generic handlers which are applied for all incoming requests.
 	var handlerFns = []HandlerFunc{
+		// Network statistics
+		setHTTPStatsHandler,
 		// Limits all requests size to a maximum fixed limit
 		setRequestSizeLimitHandler,
 		// Adds 'crossdomain.xml' policy handler to serve legacy flash clients.