Make use of ActorID.Metadata rather than separate keys infra for reception

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.

Start by tracing ActorID.Metadata, ActorTags, and the reception-key handling around checkIn(_:with:). The change is complete when a dedicated actor tag can serve as a reception key for tagged actors while the existing checkIn(_:with:) path still supports specific actor instances started later.

Written by the indexing model from the issue text.