[HttpFoundation] Added support for auto expiry in MongoDbSessionHandler

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| 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
ensureIndex is deprecated, createIndex actually does the same thing but is the new one to use.
See: http://php.net/manual/en/mongocollection.ensureindex.php