Remove lease from Event#reset_project_activity

What does this MR do?

This removes the exclusive lease used by Event#reset_project_activity in favour of conditional UPDATE queries. See dbcc623a for more information.

Why was this MR needed?

Obtaining the lease can, for whatever reason, be really slow. See https://gitlab.com/gitlab-org/gitlab-ce/issues/22473 for more information.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22473