FPRClient: fix paging API responses

Rather than try to fetch all items at once (which doesn't work for sets of results larger than 1000), rework the existing code into a simple fetch function and a generator that iterates over every item of the requested type. These are now fetched in batches of 100 and the "next" URL provided by the API response is used to page through all results.
Docstring for this? What the parameters are and how this will behave is non-obvious. (I'm thinking interactions between resource and url)