Enable state to be used by scalable actors

[mwvaughn]

The actor state variable is super-useful, but is limited to single-worker actors. Some developers have routed around this limitation by imitating the state variable with the Agave metadata service, namespacing the state by executionId. That's a viable approach, but doesn't let the actor develop any sense of collective state across its workers. What could be very 🆒 is to replace the simple state variable, which is just a Python dict, with a CRDT dictionary type.