This should create the actual `ActorRef`. `JournalFactory` is preliminary and will likely be replaced when I start working on the journal plugin API. I'll make it `private[persistence]` meanwhile.

I understand the issue of using an actor context in the wrong place but can you prevent that at all? For example an actor could reply the context to a sender, a `PromiseActorRef`, and the context is then used in the `onSuccess` callback.