[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
> Do you have citations for that? I'd be curious to know what other open-source projects are doing this.

No, to be honest I can't go any further with it that applications I have been involved in.

Some reference, but nothing hard I guess. http://stackoverflow.com/questions/7000777/mongodb-when-to-call-ensureindex

It can be a documented edit and I'll remove from this code, at least the developer is in control I guess?