Improve caching which was reverted

https://gitlab.com/gitlab-org/gitlab-ce/issues/2033

MRs: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1014

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1028

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1027

Proposal: It is difficult task. I think the best we can do it is use user_id in cache key and invalidate it when permission changes. Usually for this purposes, can be used so called cache group. And in same cases you can remove all cache keys inside certain group. I think it is the best solution.