add /api/tunnels/:id/status endpoint

Provides connected_sockets information for a tunnel.

lib/Client.js

@@ -12,6 +12,10 @@ class Client {
         this.debug = Debug('lt:Client');
     }
 
+    stats() {
+        return this.agent.stats();
+    }
+
     handleRequest(req, res) {
         this.debug('> %s', req.url);
         const opt = {