mysql module support pool query

應該是新辦本提供的功能，之前沒看到...

var mysql = require('mysql');  var pool  = mysql.createPool({    connectionLimit : 10,    host            : 'example.org',    user            : 'bob',    password        : 'secret'  });    pool.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {    if (err) throw err;      console.log('The solution is: ', rows[0].solution);  });