Thread safety on config values.

[smudge]

We have seen issues running demo mode with concurrency, and traced it to the fact that any config values on the DemoMode class itself are not thread safe and must be pre-heated before threads are spawned.

For example, this line might cause personas to return [] in a thread that had already loaded all of its persona files into memory:

    def personas
      unless instance_variable_defined?(:@personas)
        @personas = []
        auto_load_personas!
      end
      @personas
    end

The workaround for now is to preheat this config during application boot explicitly, but we should fix this more sustainably from within the gem itself.