Cache compiled API routes

I noticed that it takes 5-6 seconds for our Grape API to compile all the routes (see GitLab issue https://gitlab.com/gitlab-org/gitlab-ce/issues/35208), which is especially painful in production whenever a unicorn worker is restarted. Since API routes generally don't change that much, I think there ought to be a way to load the routes from cache.

I filed https://github.com/sinatra/mustermann/issues/69 because it seems like we'd need a way to compile and serialize the data there, but perhaps there is something Grape could be doing too.

Start by investigating how Grape compiles API routes and whether the route data can be safely loaded from a cache during Unicorn worker startup. Review the linked GitLab issue and the related Mustermann issue 69 to determine the serialization dependency. Done means establishing a workable route-cache approach with appropriate validation and tests.