Allow a better override of KeyGenerator

For instance it is only possible to set a global key generator which will be used for ALL caches.

We should allow to set a per-cache key generator as to override it in `key-generator` option to annotation.

``` php
<?php
/**
 * @Cacheable(caches="users", key="#user.getUsername(), key-generator="request_key_generator")
 */
```
