[Bundle] Enqueue\Symfony\Client\ContainerAwareProcessorRegistry expects processors to be public

[dkarlovi]

With Symfony 4, all services are private by default. But Enqueue\Symfony\Client\ContainerAwareProcessorRegistry keeps them as a set of names which it tries to get() from the container.

A better approach might be to inject tagged services or alternatively use a service locator.

[makasim]

A PR that fixes it would be great

[dkarlovi]

We should probably fix this first, then #409 since that one will make services private by default.