Allow to configure `apiLimit`, `apiQueueLimit` and `apiQueueTimeout`
This MR adds to Workhorse ability to limit number of processed requests: https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/65
It adds a 3 new options which should be exposed and configured by Omnibus:
- apiLimit - limit number of concurrent API requests,
- apiQueueLimit - limit the backlog for queueing,
- apiQueueTimeout - duration after we timeout requests if they sit too long in queue