[BC] \GearmanClient compliance, doctrine cache, symfony built-in config

Opening another PR, as there's one error in previous PR. 
Changes:
* [BC] configuration is moved to default ./app/config.yml file to leverage symfony validation, caching, parsing (requires doc update)
* lazy loading of gearman services. Do not load and parse workers unless worker or job is executed.
* removed custom cache in favor of doctrine common. Added dependency on liip/doctrine-cache-bundle to ease cache using (required doc update)
* code cleanup. Removed redundant JobCollection and WorkerCollection classes.

In fact wanted to remove GearmanClient as well, as I don't see reasons for using it instead of \GearmanClient