Add caching to fallback routes

[indykoning]

By caching fallback routes per url we can greatly reduce the response time in case of heavy controllers, like requests to external services (e.g. strapi)

Here's an example how the timeline is currently in the fallback controllers if the page is available in a later controller

Once a url has had it's controller cached the timeline looks like this

This could be further optimised by caching the response to this url to prevent the external call in the first place.
But this is the responsibility of the controller making that external call.

Customisation to the cache duration can be done via the rapidez.fallback_routes.cache_duration config key.

[royduin]

I'll merge when the PR for is master is ready 🚀