[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
I understand the reasoning for putting this here, but issuing a [createIndexes](http://docs.mongodb.org/manual/reference/command/createIndexes) command with each session write is not a good idea. This might also lead to unexpected behavior if someone on a very old MongoDB version tries to use `expiry_field` and TTL indexes aren't supported (I'm not sure if 2.0.x would ignore the option or raise an error).

I assume there will be a PR against symfony-docs to complement this. We should just point users to the [MongoDB documentation](http://docs.mongodb.org/manual/tutorial/expire-data/) so that they can create the TTL index themselves if necessary.