Make use of ActorID.Metadata rather than separate keys infra for reception #950
Description
We have ActorTags now, and can use them "give me actors tagged "team": "x"" rather than having separate reception keys -- which we needed before.
We can have a specific kind of actor tag that IS the reception key, and then we can tag actors with them, allowing for easier checking in. We must keep the checkIn(_:with:) version too though, because it allows us to check in specific actor instances for some key after they have been started already.