`_changeProperties` doesn't work correctly if you pass multiple Things in

Summary: For automation, Eric wants to be able to apply the same change to multiple (perhaps many) Things at once using _changeProperties, but only one takes effect.

The problem is pretty obvious in the code: _changeProperties is using inv.contextFirstThing.

Switching that gets much closer to right, but with a scary failure: it looks like all the right messages are being sent, but the newest one is stomping the older ones. Worse, the events are getting persisted properly -- it's just failing to record them in the live State! Which means we've got an invariant violation during race conditions, somewhere under SpaceCore.