Inits that are logically singletons (e.g. actor
location or owner) should implement this interface
to avoid runtime inconsistencies.
Duplicate instances are rejected at init-time,
allowing simpler queries when they are used.
A shared ValueActorInit<T> is introduced to reduce duplication
in the most common init cases, and an ActorInitActorReference
allow actors to be referenced by map.yaml name.
