Assign Cookie payload name with value from config

Not really a PR, more of a proposal with an example.

In session config file there's an option to set cookie name `session.cookie` which allows developer to change default session cookie name to whatever name he wants.

Then, there's a driver CookieStore which uses it's own cookie name - 'illuminate_payload'. I believe it's pretty reasonable to expect an option in config file which allows to change CookieStore's name too.

Of course, right now there's a `setPayloadName()` method available, but i really think addition of `payload` option in config file brings consistency.

Anyway, besides the change in this PR, there must be additional change to `app/confing/session.php` file:

````php
....
'payload' => 'illuminate_payload',
````