implement cursors for members, journals, funders #116

Apart from the main change, I also had to clean up offset validation. Previously, we assumed offset limit 80k for routes without cursors, and limit 10k for routes with cursors, and the validation code reflected this dependency. After introducing cursors for more routes, offset limit has nothing to do with whether the route supports cursors or not. We have simply 10k for /works and 80k for all other routes. So I had to introduce an independent offset validator.