[HttpFoundation] MongoDbSessionHandler supports auto expiry and auto index creation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11508
| License       | MIT
| Doc PR        | no

ToDo

 * [x] Fix Tests

Looking for feedback on this early PR.


This adds a config option that disables the PHP GC method call from doing anything,
It also means that the write method sets up the auto expiring index.


Ref: #11508
OK I see the concern, I was really just trying to make using this option as simple as possible and have the indexes handled automatically for you.
As I understand it, running createIndex (was ensureIndex) on each write is a fairly common approach, and is useful as it avoids having to effectively do a DB migration when you deploy such a change.

There isn't actually anything in the docs about using the MongoDbSessionHandler in general that I can add this to.