Late resolve notFound

So that it can be mutated after creation.

So fun use case:
- I want to use beefy for my dev server
- it can take a custom handler which should return true/false if it handled (or didnt't) the request
- so what I want to be able to do is have a module called say dev_handler which loads my normal router, but patches it to work as a beefy custom handler (basically making the notFound action a noop which sets state).