add /api/tunnels/:id/status endpoint

Provides connected_sockets information for a tunnel.

lib/TunnelAgent.js

@@ -39,6 +39,12 @@ class TunnelAgent extends Agent {
         this.closed = false;
     }
 
+    stats() {
+        return {
+            connectedSockets: this.connectedSockets,
+        };
+    }
+
     listen() {
         const server = this.server;
         if (this.started) {