Session driver-independent way to make sessions unique

Right now some of drivers have ways to identify their own sessions. However not all of them are willing to share that information, nor it's in a unified format.

Using `spl_object_hash` also isn't possible with `Session` class instances, because data object properties can change over time.

Here is a way to allow unique identification of each session.


Usage:

```php
$session = new Session(.....);
$unique_id = $session->getUniqueId();
```