Keyset pagination: Analyze response times on GitLab.com

With https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18584 deployed on GitLab.com (pending - 11.0 RC1 includes this, which is on staging), it's time to see how keyset pagination performs at scale.

In particular, we're interested in the following:

• Runtimes for API calls paging through a bigger collection (compare those with default pagination)
  • With custom sorting
  • With default sorting
• Look at plans to see how / if keyset pagination leverages indexes (start with projects collection)

Note, we'll have to enable this feature flag: api_keyset_pagination