Prevent deleting a tag if at least one actor references that tag's snapshot

Today, a tag or an actor may own an external snapshot.

* When a tag is created, it copies the external snapshot into it's own external storage path and
* When an actor is created from a tag, it will reference the tag's snapshot until its first Suspend call.
* An actor doesn't own its own snapshot until its first SuspendActor() call (Suspend will create/copy a snapshot)

Today, nothing prevents deleting a tag whose snapshot is referenced by running actors.

cc @juli4n