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…into dk/978-queue-implementation

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…into dk/978-queue-implementation

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The `Worker` class doesn't really need it. It only needs a
connection factory and one additional method (for detecting
whether the app is in maintenance mode), but unfortunately
type-hints the `QueueManager` class.

Laravel 6.0 will improve this, so let's hack our way around
it for now by building a minimal subclass of `QueueManager`
that only supports those two methods.

Once we upgrade to Laravel 6.0, the hack can simply be removed.

Again, Laravel wants the whole manager even though it only needs a
repository instance. Let's build a minimal workaround again until
this situation is resolved. (I submitted a PR to Laravel.)

That concept doesn't apply here - extensions will have to take
care of running migrations if needed.

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