Move to standardised fork-hooks that we can use with Puma and Unicorn

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3592

See related merge requests:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22372
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7943
• gitlab-development-kit!532 (merged)

Please note that https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22372, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7943 and this MR should all be merged concurrently.