New Security Authentication Additions

Hi guys!

This is an addition to the very nice work done by @xabbuh on #2914.

The biggest change that I'd like review on is around the user provider and the `refreshUser` method - both when we don't need session persistence and the new section where we *do* use it.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#6069)
| Applies to    | 2.4+
| Fixed tickets | #2721, #2914 

Thanks!