enforce memcached version to be 2.1.0

The signature of the `Memcached::get()` method changed with 2.2.0.
Therefore, tests fail in Symfony when mocking `Memcached` in the
`MemcachedSessionHandlerTest` of the HttpFoundation component (see
also php-memcached-dev/php-memcached#126 and
https://bugs.php.net/bug.php?id=66331).
minor #12297 enforce memcached version to be 2.1.0 (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

enforce memcached version to be 2.1.0

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9797
| License       | MIT
| Doc PR        |

The signature of the `Memcached::get()` method changed with 2.2.0.
Therefore, tests fail in Symfony when mocking `Memcached` in the
`MemcachedSessionHandlerTest` of the HttpFoundation component (see
also php-memcached-dev/php-memcached#126 and
https://bugs.php.net/bug.php?id=66331).

Commits
-------

2ac5c86 enforce memcached version to be 2.1.0
updated CHANGELOG for 2.3.21
update CONTRIBUTORS for 2.3.21
updated VERSION for 2.3.21
bumped Symfony version to 2.3.22
Improved the phpdoc for security token classes
[Security][listener] change priority of switchuser
minor #12310 Improved the phpdoc for security token classes (stof)

This PR was merged into the 2.3 branch.

Discussion
----------

Improved the phpdoc for security token classes

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

This will avoid to have Scrutinizer complaining that we pass an array of string rather than an array of RoleInterface for instance.

Commits
-------

7b6d115 Improved the phpdoc for security token classes
bug #12313 [Security][listener] change priority of switchuser (aitboudad)

This PR was merged into the 2.3 branch.

Discussion
----------

[Security][listener] change priority of switchuser

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Hi,
I have the following users:
  - Manager: Allowed to access to (/admin) and has a role ROLE_ALLOWED_TO_SWITCH
  - Partner: Allowed to access to (/partner)

When I attempt to switch to partner user I get "Access denied", well I think the switchuser listener must be registred before access listener.

Commits
-------

5f8047d [Security][listener] change priority of switchuser
disable xdebug to increase build speed