Creating a Space often gives an Unexpected Error

Summary: Two times out of three, when you try to create a Space in the production environment, you get no response, and eventually get an Unexpected Error, even though the Space has been created successfully.

This showed up during a public demo, of course.

The problem is that Space creation happens by a SpaceManager sending an InitialState() message to the new Space. But those two Actors are, 2/3 of the time, on different nodes, and the response to InitialState is a ThingFound() message -- which doesn't serialize! So the serialization fails, the ack splashes, and the request eventually times out with an error.

Fixing this properly, as part of the SpaceCore rewrite to support Apps. The ThingFound responses, which were originally a side-effect buried deep in the code, has been lifted out to the edges so that InitialState can do something different.