Change from reify to property

[digitalresistor]

We don't want to cache the factories return.

This solves #1268 (or at least part of it)

[tseaver]

This PR needs a test which fails when using '@reify' but passes with '@Property'. Also, I don't really understand the "we don't want to cache" bit: obviously somebody thought it was supposed to be cached, or we wouldn't be using '@reify'. The PR needs to add a comment (or something) explaining why caching the renderer is not the Right Thing (TM).

[mmerickel]

As I mentioned in #1268, I think this should be solved in the add_view and ViewDeriver. The @reify is not the primary problem, it's the sharing of a RendererHelper across requests.

[mcdonc]

Please don't merge this. The `@reify`` is purposeful.

[digitalresistor]

Alright, no merging. However the issue as in #1268 remains, should that be fixed as @mmerickel is suggesting with a factory that returns a RenderHelper for each request?