Implement a view cache

[slavi]

User story here is that upon restarting of Pynorama, all views are automatically refreshed. It would be best if we could leave that choice to the user, and allow users to cache their views (e.g. using a decorator). This will greatly improve the restart experience.

We already have an Arctic-based solution at AHL. We should think about whether we'd like to keep Arctic as a backend, or use something more lightweight.