Mongoose - useDb(name) not allowing more than 10 concurrent request

We are using mongoose to connect to MongoDB.Initially a pool of 100 connections is created and to connect to some other database using the mongoose method useDb

Below is the code snippet

var url = require("url");
var connectRoute = require('connect-route');
var connect = require('connect'),
    app = connect.createServer();

var mongoose = require('mongoose');
 var conn = mongoose.createConnection('mongodb://localhost:10040/first', {server: {poolSize: 100}});

 conn.on('error', console.error.bind(console, 'connection error:'));

 var Schema = mongoose.Schema;
 var MySchema = new Schema({
 user: String,
 pwd: String, roles: []
 },
 {strict: false}
 );
app.use(connectRoute(function (router){
    router.get('/get', function(req,res){
        var db2 = conn.useDb('second_DB');
        var data = db2.model('', MySchema, 'coll');
        data.update({name: "Janu"}, {$set: {"name": "test"}}, {upsert: true} , function (err, data,log) {
            //console.log(a.data.data );
            res.end(JSON.stringify(log.connectionId));
        });
    });
}));

app.listen(3000);
console.log('info','Connect server listening on port 3000 ' );

Even though Poolsize is set to 100 , when more than 10 concurrent requests are given throwing the warning as

(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.

MongoDB version - 2.6.4 Mongoose version - 3.8.12

can anyone help out on why this warning am getting ??