add semi abstract worker factory

This add a worker factory that is able to instantiate workers of any type.

Two reason why this might be bad:
- utilizing a undocumented feature (or unexpressed in the Zend\ServiceManager\FactoryInterface interface) 
- SlmQueue* must use a fully qualified class name as $requestedName will instantiate that. SlmQueue and flavor follow that convention anyway.

Why then;
- Less code repetition
- Factory can now be used to configure any worker. Consider this part of  https://github.com/juriansluiman/SlmQueue/pull/76 (worker listeners)