Session doesn't work after update from 2.2.1 to 3

Hello, I am updating from CI2.2 to CI 3.0 using the tutorial. I have done the following modifications in the config.php 
$config['sess_cookie_name'] = 'ci_sessions';
$config['sess_expiration'] = 86400; //24 hours
$config['sess_expire_on_close'] = TRUE;
$config['sess_encrypt_cookie'] = FALSE;
$config['sess_use_database'] = FALSE;
//$config['sess_table_name'] = 'ci_sessions';
$config['sess_match_ip'] = FALSE;
$config['sess_match_useragent'] = TRUE;
$config['sess_time_to_update'] = 86400; //24 hours
$config['sess_driver'] = 'database';
$config['sess_save_path'] = 'ci_sessions';

Earlier there was no ci_sessions table so I  have created one in the database.

In  autoload.php I have this line 
$autoload['libraries'] = array('database','session');

In my controller I am using   $this->load->library('session');

But I am getting error Unable to locate the specified class: Session.php 
But incase I move Session.php from Session folder to libraries I dont get any error but login page doesn't work.

I have commented session.start() function in the Session.php class as the application gave my error Severity: Warning


Message: ini_set(): A session is active. You cannot change the session module's ini settings at this time

Filename: libraries/Session.php

Line Number: 322
 
Please help me. I am new in the company without any mentor to help.