Block concurrent pipeline processings

What does this MR do?

It's possible that two builds that succeed at the same will try to concurrently process pipeline. This can lead to scenario when it will fail to trigger next stages.

This MR makes sure that process of updating is blocking.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21263