Remove all calls to call_user_func()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | lets see
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR proposes to replace all occurences of `call_user_func()` by plain PHP. This is one less function call, and one less depth level in the stack. With it, I also propose that we agree on never using `call_user_func()` in Sf 3.0, since there is always an equivalent faster PHP syntax.
this one could actually be fixed in Symfony 2.5+