use symfony 2.3 synchronized services for cmf_request_aware and emulate in symfony 2.2

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | sort of
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

The rationale for this is to have bundles that support both symfony 2.2 and 2.3. If we agree that this is sane to do, i will update the doc. It would be very nice to also test this - having the testing component bootstrapped in CoreBundle would help a lot.