[HttpFoundation] Introducing Session Caching Support for Couchbase.

This changeset introduces session caching support for Couchbase
Server in combination with its couchbase php extension.

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
License of the code: same as symfony
Documentation PR: not added yet.