Allow the QueuedJobService implementation to be configured on a per-runner basis

[robbieaverill]

If my project has two separate runners:

• DoormanRunner for immediate queue processing
• QueueRunner for multi server processing of a longer running queue

It'd be good to be able to configure the QueuedJobService implementation that is used for each runner. At the moment they always return QueuedJobService::singleton() via BaseRunner::getService().

We could change this to use something configuration based, for example:

class BaseRunner
{
    protected $serviceClass = QueuedJobService::class; // default implementation
    protected $service;

    public function setService(QueuedJobService $service)
    {
         $this->service = $service;
         return $this;
    }

    public function getService()
    {
        if (!$this->service) {
            $this->service = Injector::inst()->get($this->serviceClass);
        }
        return $this->service;
}

SilverStripe\Core\Injector\Injector:
  MyQueueRunner:
    properties:
      Service: '%$MyCustomQueuedJobService'

There's other places where the service class is loaded too, so we'd also need to consider how to load the right job service in various points of the API - this example would only work when loaded from runners, and we don't want to redefine DI config in multiple places.

It's also worth noting that the way the config works at the moment is that the QueuedJobService class defines the runner as a dependency of it, so really BaseRunner::getService() should be returning the class that it's injected into.