Peers: optional latency check with per-peer setting

The server pings a peer's tunnel address every 30 s and shows the median
of the last 5 minutes in the peer list (with a 1-hour sparkline) and a
24-hour chart on the peer page. Off by default; "active" pings only
while the device sends traffic, "always" keeps the tunnel up.

main.go

@@ -208,9 +208,10 @@ func run(configPath string) error {
 	}
 
 	var wg sync.WaitGroup
-	wg.Add(3)
+	wg.Add(4)
 	go func() { defer wg.Done(); recon.Run(stop) }()
 	go func() { defer wg.Done(); stats.Run(stop) }()
+	go func() { defer wg.Done(); stats.RunPings(stop) }()
 	go func() { defer wg.Done(); geo.Run(stop) }()
 	go func() {
 		t := time.NewTicker(10 * time.Minute)