[HttpFoundation] MongoDbSessionHandler supports auto expiry via configurable expiry_field
feature #11668 [HttpFoundation] MongoDbSessionHandler supports auto expiry via configurable expiry_field (catchamonkey)

This PR was merged into the 2.6-dev branch.

Discussion
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[HttpFoundation] MongoDbSessionHandler supports auto expiry via configurable expiry_field

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11508
| License       | MIT
| Doc PR        | no

This PR applies #11510 to master instead of 2.3

Should be merged on master:
- [x] after https://github.com/symfony/symfony/pull/11667 is merged to 2.3
- [x] and after 2.3 is merged back to master

Commits
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beca900 [HttpFoundation] MongoDbSessionHandler supports auto expiry via configurable expiry_field