WIP: Expose per-project pipeline ID ($CI_PIPELINE_IID)

What does this MR do?

This MR creates new column iid on ci_pipelines and expose it as $CI_PIPELINE_IID. This iid is per-project basis.

Are there points in the code the reviewer needs to double check?

Does this migration work on production(GitLab.com)?

Why was this MR needed?

Originally demanded at https://gitlab.com/gitlab-org/gitlab-ce/issues/3691. It got about 50 upvotes.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/3691

/cc @ayufan @bikebilly @grzesiek @godfat