Rename correlation-id structured logging field to correlation_id

Currently Workhorse uses the field correlation-id. Ruby's structured logging fields all use underscores instead of dashes and keeping consistency in this field is highly desired. For this reason, following discussion on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22844?diff_id=28917866&start_sha=3fc797b8270ab74e15cf0e0def3ca25a12e041a4#note_122979465 between @stanhu, @ayufan and myself, it was decided to rename this field in workhorse to correlation_id.

Correlation IDs are currently only logged by Workhorse and no other systems, making them fairly useless. For this reason, this change should not have any negative impact and very little risk.