wpdb class : provide a disconnect / connect to db functions

[arena]

some hosts automatically disconnect mysql after a short while.
I think it could be interesting to have in wpdb a disconnect and connect functions.

Related: #5932

Hi ocean90

the code i am currently using

class data_base
{
	function mysql_disconnect()
	{
		global $wpdb;
		mysql_close($wpdb->dbh);
	}

	function mysql_connect()
	{
		global $wpdb;
		$wpdb->__construct(DB_USER, DB_PASSWORD, DB_NAME, DB_HOST);
	}
}

but still have some "mysql_real_escape_string(): nn is not a valid MySQL- Link resource" on some webhosts after disconnecting/connecting !