Clean up JiraService
What does this MR do?
It implements the steps proposed in https://gitlab.com/gitlab-org/gitlab-ce/issues/24967 that make
JiraService code more understandable and aligned with other project services. It also prevents making unnecessary calls to JIRA API by
ProjectServiceWorker . Also, https://gitlab.com/gitlab-org/gitlab-ce/issues/24777 will no longer happen after these changes.
Are there points in the code the reviewer needs to double check?
Does this MR meet the acceptance criteria?
- Changelog entry added
[ ] Documentation created/updated [ ] API support added
[ ] Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
Branch has no merge conflicts with
master(if it does - rebase it please)
- Squashed related commits together